The average American adult consumes about 300 mg of caffeine per day.  That’s equivalent to the caffeine content of nearly three cups of coffee (we have previously covered how much caffeine is safe to consume.
With so much caffeine being consumed by so many people, it’s worth taking a look at potential downsides. We recently added some side effect data to our caffeine page, and here’s a quick summary of what we found:
It’s pretty clear that caffeine increases blood pressure in the short term, and this spike seems to be stronger in people prone to hypertension. This acute response is also much more pronounced in people who don’t normally ingest caffeine. But these acute effects wear off after around 4 hours.
What’s less clear is caffeine’s long-term effects on blood pressure.  The overall long-term effects are inconclusive,  with the big picture being obscured in part to different responses seen with pure caffeine ingestion and studies involving caffeinated beverages such as coffee. One meta-analysis of studies with a median length of 42 days found that pure caffeine bumps up blood pressure by about 4/2 mmHg, whereas coffee intake bumped systolic blood pressure by about 1 mmHg with no significant effect on the diastolic number. 
But a few months is hardly long-term data. And another meta-analysis of randomized controlled trials with cofffee have found no association between the risk of developing hypertension and coffee consumption,  which concords with observational evidence suggesting no relationship between cardiovascular disease and caffeinated coffee consumption. 
So, what’s the take-away here? It’s hard to say. There are a lot of differences across studies, which may explain the varying results.  But, however much you may get sick of hearing this phrase, it rings true in this case: more research is needed.
Caffeine consumption likely causes a short-term spike in blood pressure after consumption, with the spike being more pronounced in those who don’t normally ingest caffeine and in those with hypertension. However, the evidence concerning the long-term effects of caffeine and caffeinated beverages on blood pressure is mixed.
Caffeine may also affect the pressure inside of the eye. A meta-analysis looking at acute caffeine ingestion found that it significantly raises the pressure inside the eye of people with glaucoma, as well as others with pre-existing high eye pressure (ocular hypertension). People with normal eye pressure, however, were unaffected. 
This concords with observational evidence which also found an association between eye pressure and caffeine intake in people with open-angle glaucoma, although this study didn’t find an association in those with ocular hypertension. 
Caffeine may raise eye pressure, but only in those who have pre-existing eye conditions like glaucoma.
One study on people who consumed more than three cups of coffee per day found that an acute dose of caffeine mixed with water lowered the pressure exerted by the lower esophageal sphincter, which holds the contents of the stomach down.  This could in theory lead to acid reflux, since the contents of the stomach could more easily come up. It’s interesting to note that this effect was seen in people who regularly ingest caffeine.
Caffeine can promote conditions leading to acid reflux, even in people who regularly ingest caffeine.
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