Chronic non-cycled caffeine consumption (in the form of coffee and tea, both of which contain many bioactive components in addition to caffeine) is associated with a reduced risk of several diseases, including Parkinson’s disease, type 2 diabetes, and chronic liver disease.[reference|url=https://pubmed.ncbi.nlm.nih.gov/32706535|title=Coffee, Caffeine, and Health.|published=2020-07-23|authors=van Dam RM, Hu FB, Willett WC|journal=N Engl J Med|] Caffeine consumption has also been shown to enhance performance on attention tasks, regardless of the participants’ caffeine consumption habits.[reference|url=https://pubmed.ncbi.nlm.nih.gov/23241646|title=Caffeine as an attention enhancer: reviewing existing assumptions.|published=2013-Jan|authors=Einöther SJ, Giesbrecht T|journal=Psychopharmacology (Berl)|] Clearly, cycling isn’t needed to obtain these benefits. So why cycle? The question usually comes up in the context of exercise performance enhancement.

Supplementation with 3–6 mg of caffeine per kg of body weight before exercise has been consistently shown to enhance performance over a wide range of exercise intensities and durations, with beneficial effects on aerobic exercise performance, muscular endurance and strength, sprinting, jumping, and throwing performance.[reference|url=https://pubmed.ncbi.nlm.nih.gov/33388079|title=International society of sports nutrition position stand: caffeine and exercise performance|published=2021 Jan 2|authors=Nanci S Guest, Trisha A VanDusseldorp, Michael T Nelson, Jozo Grgic, Brad J Schoenfeld, Nathaniel D M Jenkins, Shawn M Arent, Jose Antonio, Jeffrey R Stout, Eric T Trexler, Abbie E Smith-Ryan, Erica R Goldstein, Douglas S Kalman, Bill I Campbell|journal=J Int Soc Sports Nutr|]

Caffeine elicits an ergogenic effect (i.e., enhances exercise performance) primarily by affecting the central nervous system. Caffeine blocks adenosine receptors in the brain, resulting in increased release of neurotransmitters such as dopamine and norepinephrine, and thus increased alertness and focus. It can also reduce pain and perceived exertion during exercise.

It seems to many habitual caffeine users that habitual use reduces, or even eliminates, the ergogenic effect of acute caffeine ingestion. The rationale is that habitual caffeine use increases the number of adenosine receptors in the brain, and as such, reduces the adenosine-blocking effect of caffeine. This has been shown in rodents,[reference|url=https://pubmed.ncbi.nlm.nih.gov/6291335|title=Adenosine actions and adenosine receptors after 1 week treatment with caffeine.|published=1982-Jun|authors=Fredholm BB|journal=Acta Physiol Scand|][reference|url=https://pubmed.ncbi.nlm.nih.gov/2110844|title=Effects of long-term theophylline treatment on adenosine A1-receptors in rat brain: autoradiographic evidence for increased receptor number and altered coupling to G-proteins.|published=1990-Jan-22|authors=Fastbom J, Fredholm BB|journal=Brain Res|] but has yet to be studied in humans. Evidence from human studies does suggest that habitual caffeine use can blunt some aspects of the physiological response to caffeine, such as an increase in plasma epinephrine levels.[reference|url=https://pubmed.ncbi.nlm.nih.gov/8226485|title=Caffeine metabolism and epinephrine responses during exercise in users and nonusers.|published=1993-Aug|authors=Van Soeren MH, Sathasivam P, Spriet LL, Graham TE|journal=J Appl Physiol (1985)|][reference|url=https://pubmed.ncbi.nlm.nih.gov/7009653|title=Tolerance to the humoral and hemodynamic effects of caffeine in man.|published=1981-Apr|authors=Robertson D, Wade D, Workman R, Woosley RL, Oates JA|journal=J Clin Invest|]

In further support of the idea that athletes should cycle caffeine in order to maximize its ergogenic effect, some studies indicate that when individuals with low habitual caffeine intake (< 75 mg/day) consume 3 mg of caffeine per kg of body weight daily for 20–28 days, the ergogenic effect of caffeine is reduced.[reference|url=https://pubmed.ncbi.nlm.nih.gov/27762662|title=Chronic ingestion of a low dose of caffeine induces tolerance to the performance benefits of caffeine|published=2017 Oct|authors=Ross Beaumont, Philip Cordery, Mark Funnell, Stephen Mears, Lewis James, Phillip Watson|journal=J Sports Sci|][reference|url=https://pubmed.ncbi.nlm.nih.gov/30673725|title=Time course of tolerance to the performance benefits of caffeine|published=2019 Jan 23|authors=Lara B, Ruiz-Moreno C, Salinero JJ, Del Coso J|journal=PLoS One|]

However, in a 2022 meta-analysis of 59 studies that investigated the effect of acute supplementation with caffeine on exercise performance in habitual caffeine consumers, it was found that acute supplementation with caffeine improved performance during endurance, power, and strength exercises.[reference|url=https://pubmed.ncbi.nlm.nih.gov/35536449|title=Can I Have My Coffee and Drink It? A Systematic Review and Meta-analysis to Determine Whether Habitual Caffeine Consumption Affects the Ergogenic Effect of Caffeine|published=2022 May 10|authors=Arthur Carvalho, Felipe Miguel Marticorena, Beatriz Helena Grecco, Gabriel Barreto, Bryan Saunders|journal=Sports Med|] Moreover, the performance-enhancing effects of an acute dose of caffeine were independent of whether or not that dose was greater or smaller than participants' habitual consumption, as well as being independent of sex or training status.

The results of this meta-analysis indicate that caffeine does not lose its ergogenicity with chronic use; that is, even with daily caffeine consumption, ingesting some caffeine about an hour before exercise will still have a positive impact on performance in people who benefit from caffeine (which is not everyone). But is it possible that skipping a cup of coffee today will enhance the effects of a cup of coffee on exercise performance tomorrow? Probably not.

Two separate studies reported that abstaining from caffeine for four days prior to an exercise test did not enhance the effect of acute supplementation with caffeine,[reference|url=https://pubmed.ncbi.nlm.nih.gov/21279864|title=Caffeine withdrawal and high-intensity endurance cycling performance|published=2011 Mar|authors=Irwin C, Desbrow B, Ellis A, O'Keeffe B, Grant G, Leveritt M|journal=J Sports Sci|][reference|url=https://pubmed.ncbi.nlm.nih.gov/9760346|title=Effect of caffeine on metabolism, exercise endurance, and catecholamine responses after withdrawal.|published=1998-Oct|authors=Van Soeren MH, Graham TE|journal=J Appl Physiol (1985)|] and shorter withdrawal periods (24–48 hours) don’t appear to be useful either.[reference|url=https://pubmed.ncbi.nlm.nih.gov/35536449|title=Can I Have My Coffee and Drink It? A Systematic Review and Meta-analysis to Determine Whether Habitual Caffeine Consumption Affects the Ergogenic Effect of Caffeine|published=2022 May 10|authors=Arthur Carvalho, Felipe Miguel Marticorena, Beatriz Helena Grecco, Gabriel Barreto, Bryan Saunders|journal=Sports Med|] Therefore, the available evidence does not support the common practice of utilizing a caffeine withdrawal period before an important training session or competition to maximize caffeine’s ergogenic effect. In fact, doing so may even be detrimental, as a four-day withdrawal period was found to result in a number of side effects, including headaches, fatigue, impaired focus, and a lack of motivation.

It remains to be determined whether a longer withdrawal period can boost caffeine’s ergogenic effect, but considering the fact that acute supplementation with caffeine enhances exercise performance in habitual users, and abstaining from caffeine tends to result in unfavorable symptoms, the juice is unlikely to be worth the squeeze if the purpose of a caffeine withdrawal period is solely to maximize caffeine’s ergogenic effect on a specific day.

Caffeine is the most widely consumed psychoactive drug in the world, largely due to its mood-enhancing and stimulatory effects.

Despite widespread consumption, few people are actually aware of how caffeine works in the body.

We thought you might be interested in learning exactly how caffeine works in your body after you ingest it and it enters the brain.

But first: how adenosine makes you feel sleepy

The key player here is adenosine.

If you remember your high school biology, think of the lock-and-key model.

Adenosine is a key that opens up a variety of locks, with the locks actually being receptors in the brain.

Once adenosine (the key) locks into a certain receptor (the lock) in the brain, it has a unique effect on the brain.

There are a host of different receptors in your brain, so different ones have different effects. The one we’re interested in is the A1 receptor. Once adenosine locks with the A1 receptor, it promotes muscle relaxation and sleepiness, which is why people get tired as the day progresses.

Furthermore, adenosine can bind to the A2A receptor. When it binds, this interferes with the release of mood-improving neurotransmitters, such as dopamine.

Adenosine itself is produced primarily from physical work and intensive brain use. Thus, over the course of the day, your body accumulates adenosine.

If only there was something that could get in the way of adenosine from locking into the A1 receptor...

Adenosine is one of the best-known sleep-regulating molecules. Located in your central nervous system, it helps get you sleepy as the day winds down, among other effects.

What caffeine does in your brain

Before caffeine

When you first wake up, your body has metabolized away the adenosine molecules. You’re a bit groggy, but you’re waking up.

Ingesting Caffeine

Most people initially drink caffeine in the form of a beverage. It’s absorbed in the small intestines within an hour, and becomes available throughout the blood and most parts of the body, including your brain.

As it starts entering your brain, it starts competing with adenosine.

Peak Concentration

Blood concentrations of caffeine tend to peak within two hours, which also means that brain concentrations of caffeine are at their peaks. The caffeine in your brain is competing with adenosine and preventing it from binding to A1 receptors. This is what gives you a jolt of wakefulness.

To be precise, the caffeine doesn’t actually lock in with the A1 receptor. It’s more like something that gets in the way and occupies the lock, rather than actually unlocking it.

It similarly gets in the way of the A2A receptor, which can help promote the release of dopamine and glutamate (and make you feel good after you drink coffee)!

Decreasing caffeine

Eventually, caffeine molecules will unbind from the adenosine receptors (as all molecules generally do).

Most of caffeine is metabolized through the CYP1A1/2 enzymes into various substances such as araxanthine, theobromine, and theophylline.

The half-life (the amount of time it takes for the concentration of a substance to be halved) of caffeine in the body ranges from three to ten hours depending on the amount of CYP1A1 in the body, which varies from individual to individual.

Feeling sleepy again

By early evening, most of the caffeine from your morning cup of coffee has metabolized. There are significantly fewer caffeine molecules occupying the A1 receptors, so adenosine starts binding to them.

This starts promoting muscle relaxation and sleepiness, and that’s why you start feeling sleepy.

When you go to sleep and your body starts recovering, the adenosine molecules are metabolized. This is why sleep is so important - one of the issues with a lack of sleep is the increase in adenosine molecules. This then takes us back to the "Before Caffeine" step.

Of course, you can always attempt to drink a larger dose of caffeine at one sitting, or drink caffeine multiple times during the day to keep sleepiness at bay. But that’s not really a sustainable strategy.

Caffeine allows people to remain awake by competing with a molecule that promotes sleepiness called adenosine. Caffeine has a similar shape to adenosine and prevents it from binding to its receptors.

For more information, check out our in-depth caffeine page.

The energy drink market is huge. In 2016, it was valued at $43 billion globally and $12 billion just in the United States, where it is expected to nearly double by 2025.

Figure 1: Energy drink market in the United States (revenue in billion dollars)

Between 2007 and 2017, population surveys reported that 58% of young Canadians,[reference|url=https://pubmed.ncbi.nlm.nih.gov/28755982|title=Use and Perceptions of Caffeinated Energy Drinks and Energy Shots in Canada|published=2017 Dec|authors=Wiggers D, Reid JL, White CM, Hammond D|journal=Am J Prev Med|] 53% of US military personnel,[reference|url=https://pubmed.ncbi.nlm.nih.gov/25293546|title=Energy drink and energy shot use in the military|published=2014 Oct|authors=Stephens MB, Attipoe S, Jones D, Ledford CJ, Deuster PA|journal=Nutr Rev|] 51% of US college students,[reference|url=https://pubmed.ncbi.nlm.nih.gov/17974021|title=A survey of energy drink consumption patterns among college students|published=2007 Oct 31|authors=Malinauskas BM, Aeby VG, Overton RF, Carpenter-Aeby T, Barber-Heidal K|journal=Nutr J|] and 35% of New Zealand adolescents[reference|url=https://pubmed.ncbi.nlm.nih.gov/28905482|title=Energy drink consumption among New Zealand adolescents: Associations with mental health, health risk behaviours and body size|published=2017 Sep 14|authors=Utter J, Denny S, Teevale T, Sheridan J|journal=J Paediatr Child Health|] consumed energy drinks. In other words, a good chunk of the young population.

So what makes energy drinks so popular among the young? The simple desire for an energy boost is a major factor, of course: energy drinks have become the coffee of a new generation, thanks to successful marketing campaigns[reference|url=https://pubmed.ncbi.nlm.nih.gov/29151382|title=Exposure and perceptions of marketing for caffeinated energy drinks among young Canadians|published=2018 Feb|authors=Hammond D, Reid JL|journal=Public Health Nutr|][reference|url=https://pubmed.ncbi.nlm.nih.gov/25754297|title=Patterns of energy drink advertising over US television networks|published=2015 Mar-Apr|authors=Emond JA, Sargent JD, Gilbert-Diamond D|journal=J Nutr Educ Behav|] and the resultant peer pressure. Other factors include availability, cost, and taste.[reference|url=https://pubmed.ncbi.nlm.nih.gov/28987203|title=Informing Intervention Strategies to Reduce Energy Drink Consumption in Young People: Findings From Qualitative Research|published=2017 Oct|authors=Francis J, Martin K, Costa B, Christian H, Kaur S, Harray A, Barblett A, Oddy WH, Ambrosini G, Allen K, Trapp G|journal=J Nutr Educ Behav|] Negative health effects seem a common experience[reference|url=https://pubmed.ncbi.nlm.nih.gov/29335277|title=Adverse effects of caffeinated energy drinks among youth and young adults in Canada: a Web-based survey|published=2018 Jan 9|authors=Hammond D, Reid JL, Zukowski S|journal=CMAJ Open|] but a small deterrent.

A growing number of young people consume energy drinks. Among them, some have concerns about the potential harmful effects of regular use, but other factors (marketing, availability, cost, taste, and the desire for an energy boost) weigh more in their purchase decisions.

What’s in an energy drink?

No two products are exactly the same, but common ingredients include caffeine, taurine, B vitamins (B₃, B₆, B₁₂, inositol), herbs (e.g., ginseng, guarana), and of course some sugar or artificial sweetener.

While we could review each ingredient to evaluate its safety, that approach would cause us to miss the forest for the trees: ingredients consumed together usually interact.[reference|url=https://pubmed.ncbi.nlm.nih.gov/15549275|title=Cognitive and physiological effects of an "energy drink": an evaluation of the whole drink and of glucose, caffeine and herbal flavouring fractions|published=2004 Nov|authors=Scholey AB, Kennedy DO|journal=Psychopharmacology (Berl)|][reference|url=https://pubmed.ncbi.nlm.nih.gov/22819803|title=Differential cognitive effects of energy drink ingredients: caffeine, taurine, and glucose|published=2012 Oct|authors=Giles GE, Mahoney CR, Brunyé TT, Gardony AL, Taylor HA, Kanarek RB|journal=Pharmacol Biochem Behav|] For example, a double-blind, randomized crossover trial found that an energy drink could boost cognitive performance even though separately consuming its ingredients (caffeine, glucose, ginseng, and ginkgo biloba) did not.[reference|url=https://pubmed.ncbi.nlm.nih.gov/15549275|title=Cognitive and physiological effects of an "energy drink": an evaluation of the whole drink and of glucose, caffeine and herbal flavouring fractions|published=2004 Nov|authors=Scholey AB, Kennedy DO|journal=Psychopharmacology (Berl)|] Another study found that taurine opposed some of caffeine’s effects, so that taking both ingredients together resulted in less vigor and more headaches than consuming caffeine alone.[reference|url=https://pubmed.ncbi.nlm.nih.gov/22819803|title=Differential cognitive effects of energy drink ingredients: caffeine, taurine, and glucose|published=2012 Oct|authors=Giles GE, Mahoney CR, Brunyé TT, Gardony AL, Taylor HA, Kanarek RB|journal=Pharmacol Biochem Behav|]

That said, two compounds do deserve special attention.

Caffeine

Caffeine is the main stimulant in energy drinks. Most popular energy drinks contain some 150 mg of caffeine per 16 fl. oz., not counting the caffeine content of other ingredients, such as guarana or kola nut. In other words, the caffeine content of an energy drink is about 470% that of a can of Coke (32 mg in 12 fl. oz.) and 160% that of a cup of coffee (95 mg in 8 fl. oz). A caffeine intake of less than 400 mg/day is considered safe, but the actual safety threshold depends on individual factors such as genetics, health status, and circadian rhythms.[reference|url=https://pubmed.ncbi.nlm.nih.gov/28603504|title=The Safety of Ingested Caffeine: A Comprehensive Review|published=2017 May 26|authors=Temple JL, Bernard C, Lipshultz SE, Czachor JD, Westphal JA, Mestre MA|journal=Front Psychiatry|]

Take CYP1A2, notably. This cytochrome P450 enzyme is responsible for clearing caffeine and many other drugs from the body. Both genetics and lifestyle factors (smoking, dietary habits, contraceptive use …) influence its activity and can therefore alter caffeine metabolism.[reference|url=https://pubmed.ncbi.nlm.nih.gov/16128905|title=Assessment of CYP1A2 activity in clinical practice: why, how, and when?|published=2005 Sep|authors=Faber MS, Jetter A, Fuhr U|journal=Basic Clin Pharmacol Toxicol|] Similarly, genetic variability in adenosine receptors (caffeine’s molecular targets) can affect sleep[reference|url=https://pubmed.ncbi.nlm.nih.gov/17329997|title=A genetic variation in the adenosine A2A receptor gene (ADORA2A) contributes to individual sensitivity to caffeine effects on sleep|published=2007 May|authors=Rétey JV, Adam M, Khatami R, Luhmann UF, Jung HH, Berger W, Landolt HP|journal=Clin Pharmacol Ther|] and predispose someone to caffeine-induced anxiety.[reference|url=https://pubmed.ncbi.nlm.nih.gov/12825092|title=Association between A2a receptor gene polymorphisms and caffeine-induced anxiety|published=2003 Sep|authors=Alsene K, Deckert J, Sand P, de Wit H|journal=Neuropsychopharmacology|]

Sugar

In themselves, carbohydrates do not cause obesity or related health problems. That said, the consumption of sugary drinks often leads to an increase in caloric intake; sugary drinks can thus contribute to obesity and related health problems, such as diabetes, metabolic syndrome, and fatty liver disease, especially in a society that favors sitting over moving.[reference|url=https://pubmed.ncbi.nlm.nih.gov/16895873|title=Intake of sugar-sweetened beverages and weight gain: a systematic review|published=2006 Aug|authors=Malik VS, Schulze MB, Hu FB|journal=Am J Clin Nutr|][reference|url=https://pubmed.ncbi.nlm.nih.gov/24652725|title=Dietary sugar and body weight: have we reached a crisis in the epidemic of obesity and diabetes?: health be damned! Pour on the sugar|published=2014 Apr|authors=Bray GA, Popkin BM|journal=Diabetes Care|]

Different energy drinks contain different dosages of different ingredients, all of which have the potential to interact. For that reason, it is prudent to evaluate the safety of each energy drink as a whole rather than only on the basis of its individual ingredients. Still, it should be noted that added sugars might be uniquely harmful in the modern environment and that the metabolism of caffeine, the primary stimulant in energy drinks, exhibits high levels of interindividual variability.

Do energy drinks cause harm?

They definitely can.

Between October 1, 2010, and September 30, 2011, the US National Poison Data System (NPDS) received 2.3 million calls. Of the 4,854 (0.2%) calls related to energy drinks,[reference|url=https://pubmed.ncbi.nlm.nih.gov/23879181|title=An analysis of energy-drink toxicity in the National Poison Data System|published=2013 Aug|authors=Seifert SM, Seifert SA, Schaechter JL, Bronstein AC, Benson BE, Hershorin ER, Arheart KL, Franco VI, Lipshultz SE|journal=Clin Toxicol (Phila)|] nearly two-thirds were linked to contamination by unknown additives. The major adverse events not linked to unknown additives included seizures and cardiovascular problems such as dysrhythmia (abnormal heart rhythm) and tachypnea (abnormally rapid breathing). The events reported were most severe when the energy drinks had been consumed with alcohol.

The cardiovascular effects of energy drinks deserve special attention. Caffeine primarily affects the blood vessels, whereas the other ingredients primarily affect the heart.[reference|url=https://pubmed.ncbi.nlm.nih.gov/25716925|title=The blood pressure-elevating effect of Red Bull energy drink is mimicked by caffeine but through different hemodynamic pathways|published=2015 Feb 25|authors=Miles-Chan JL, Charrière N, Grasser EK, Montani JP, Dulloo AG|journal=Physiol Rep|] Overall, energy drinks are believed to cause short-term adverse changes characterized by an increase in the heart’s workload and a reduction in blood flow to the brain.[reference|url=https://pubmed.ncbi.nlm.nih.gov/27633110|title=Energy Drinks and Their Impact on the Cardiovascular System: Potential Mechanisms|published=2016 Sep 15|authors=Grasser EK, Miles-Chan JL, Charrière N, Loonam CR, Dulloo AG, Montani JP|journal=Adv Nutr|] These effects are caused by caffeine and its interactions with sugar, and to a lesser extent by auxiliary substances, such as taurine.

Figure 2: Potential effects of an energy drink (Red Bull) and its constituents on blood flow and pressure

Some case reports associate energy drinks and atrial fibrillation;[reference|url=https://pubmed.ncbi.nlm.nih.gov/21247417|title=Atrial fibrillation in healthy adolescents after highly caffeinated beverage consumption: two case reports|published=2011 Jan 19|authors=Di Rocco JR, During A, Morelli PJ, Heyden M, Biancaniello TA|journal=J Med Case Rep|] they mention higher doses than were used in the studies that found no significant association between energy drinks and atrial fibrillation.[reference|url=https://pubmed.ncbi.nlm.nih.gov/21106919|title=Dietary factors and incident atrial fibrillation: the Framingham Heart Study|published=2011 Feb|authors=Shen J, Johnson VM, Sullivan LM, Jacques PF, Magnani JW, Lubitz SA, Pandey S, Levy D, Vasan RS, Quatromoni PA, Junyent M, Ordovas JM, Benjamin EJ|journal=Am J Clin Nutr|] Other cardiovascular changes associated with energy drinks include ST elevation,[reference|url=https://pubmed.ncbi.nlm.nih.gov/25215082|title=ST elevation myocardial infarction in a young patientafter ingestion of caffeinated energy drink and ecstasy|published=2012|authors=Hanan Israelit S, Strizevsky A, Raviv B|journal=World J Emerg Med|] cardiomyopathy,[reference|url=https://pubmed.ncbi.nlm.nih.gov/22451608|title=Reverse Takotsubo cardiomyopathy associated with the consumption of an energy drink|published=2012 Mar 27|authors=Kaoukis A, Panagopoulou V, Mojibian HR, Jacoby D|journal=Circulation|] cardiac arrest,[reference|url=https://pubmed.ncbi.nlm.nih.gov/22056042|title=Cardiac arrest due to long QT syndrome associated with excessive consumption of energy drinks|published=2012 Jul 26|authors=Rottlaender D, Motloch LJ, Reda S, Larbig R, Hoppe UC|journal=Int J Cardiol|] coronary vasospasm,[reference|url=https://pubmed.ncbi.nlm.nih.gov/19120009|title=Cardiac arrest in a young man following excess consumption of caffeinated "energy drinks"|published=2009 Jan 5|authors=Berger AJ, Alford K|journal=Med J Aust|] acute coronary thrombosis,[reference|url=https://pubmed.ncbi.nlm.nih.gov/22431887|title=Left main coronary artery acute thrombosis related to energy drink intake|published=2012 Mar 20|authors=Benjo AM, Pineda AM, Nascimento FO, Zamora C, Lamas GA, Escolar E|journal=Circulation|] and aortic aneurysm dissection.[reference|url=https://pubmed.ncbi.nlm.nih.gov/23914511|title=High-energy drinks may provoke aortic dissection|published=2013 May|authors=Jonjev ZS, Bala G|journal=Coll Antropol|]

Energy drinks have been linked to acute poisoning, mainly because of contaminants in the drinks. However, uncontaminated energy drinks have also been linked to poisoning through adverse effects on the cardiovascular system.

Who’s at risk?

Women who plan to get pregnant should avoid taking too much caffeine: women who consume more than 400 mg/day are 11% more likely to abort spontaneously than women who consume less than 50 mg/day.[reference|url=https://pubmed.ncbi.nlm.nih.gov/27573467|title=Pre-pregnancy caffeine and caffeinated beverage intake and risk of spontaneous abortion|published=2016 Aug 29|authors=Gaskins AJ, Rich-Edwards JW, Williams PL, Toth TL, Missmer SA, Chavarro JE|journal=Eur J Nutr|]

And it doesn’t stop there. When consumed during pregnancy, caffeine can interfere with the brain development of the fetus and thus cause lasting cognitive impairment.[reference|url=https://pubmed.ncbi.nlm.nih.gov/23926202|title=Adenosine receptor antagonists including caffeine alter fetal brain development in mice|published=2013 Aug 7|authors=Silva CG, Métin C, Fazeli W, Machado NJ, Darmopil S, Launay PS, Ghestem A, Nesa MP, Bassot E, Szabó E, Baqi Y, Müller CE, Tomé AR, Ivanov A, Isbrandt D, Zilberter Y, Cunha RA, Esclapez M, Bernard C|journal=Sci Transl Med|] It doesn’t help that being pregnant alters caffeine metabolism in a way that increases caffeine’s half-life and, consequently, increases fetal exposure.[reference|url=https://pubmed.ncbi.nlm.nih.gov/6954898|title=The effect of pregnancy on the pharmacokinetics of caffeine|published=1982|authors=Knutti R, Rothweiler H, Schlatter C|journal=Arch Toxicol Suppl|]

And it doesn’t stop there, either. Children and adolescents may be another at-risk population: due to their small size, they have a lower safety threshold for many of the ingredients in energy drinks. That NPDS study referenced earlier showed that half of the non-alcoholic energy drink incidents were in children younger than 6 years, and an additional 10.5% in children aged 6–12 years.[reference|url=https://pubmed.ncbi.nlm.nih.gov/23879181|title=An analysis of energy-drink toxicity in the National Poison Data System|published=2013 Aug|authors=Seifert SM, Seifert SA, Schaechter JL, Bronstein AC, Benson BE, Hershorin ER, Arheart KL, Franco VI, Lipshultz SE|journal=Clin Toxicol (Phila)|] It has been recommended that kids limit their daily caffeine intake to less than 2.5 mg per kilogram of body weight (1.13 mg/lb)[reference|url=https://pubmed.ncbi.nlm.nih.gov/25099503|title=The suitability of caffeinated drinks for children: a systematic review of randomised controlled trials, observational studies and expert panel guidelines|published=2014 Aug|authors=Ruxton CH|journal=J Hum Nutr Diet|] — for the average 10-year-old, that means less than 80 mg, so about 8 fl. oz. (237 mL) of most energy drinks.

Are there benefits?

Several randomized controlled trials in adults have shown energy drinks to benefit mental performance.[reference|url=https://pubmed.ncbi.nlm.nih.gov/11665810|title=The effects of red bull energy drink on human performance and mood|published=2001|authors=Alford C, Cox H, Wescott R|journal=Amino Acids|][reference|url=https://pubmed.ncbi.nlm.nih.gov/21186930|title=Acute effects of a glucose energy drink on behavioral control|published=2010 Dec|authors=Howard MA, Marczinski CA|journal=Exp Clin Psychopharmacol|][reference|url=https://pubmed.ncbi.nlm.nih.gov/18470842|title=The effects of energy drinks on cognitive performance|published=2008|authors=van den Eynde F, van Baelen PC, Portzky M, Audenaert K|journal=Tijdschr Psychiatr|] The fatigue-fighting effects of caffeine could also be considered a benefit under the right circumstances.[reference|url=https://pubmed.ncbi.nlm.nih.gov/17950009|title=Caffeine: sleep and daytime sleepiness|published=2008 Apr|authors=Roehrs T, Roth T|journal=Sleep Med Rev|] Moreover, a meta-analysis found that energy drink consumption improved muscle strength and endurance, performance on endurance exercise tests, jumping, and sport-specific actions.[reference|url=https://pubmed.ncbi.nlm.nih.gov/27757591|title=Acute effects of caffeine-containing energy drinks on physical performance: a systematic review and meta-analysis|published=2017 Feb|authors=Souza DB, Del Coso J, Casonatto J, Polito MD|journal=Eur J Nutr|]

In addition to fighting fatigue, energy drinks boost cognition and exercise performance, but you should weigh the benefits against the potential risks. Pregnant women and fetuses are especially vulnerable to the adverse effects of energy drinks, as are children and adolescents due to their small size.

So, are energy drinks bad for you?

Case reports have associated energy drinks with adverse health effects, most commonly related to the cardiovascular system. Caffeine content is probably the main culprit, but sugars also play a part, and so could other ingredients — or their combination. As with all things, energy drinks can do harm but their actually doing so depends on many factors, such as the amount being consumed and the health state of the consumer.