Hibiscus Sabdariffa (Roselle) is a supplemental herb that is derived from the plant's calyces, which are the collection of sepals separating the blooming flower from the stem. The calyces have traditionally been steeped into tea where the anthocyanins (red-blue pigmentation) is steeped into the water and drank for medicinal purposes.
Although it has a variety of claims medicinally, it appears to have evidence to support its role in reducing blood pressure in persons with elevated blood pressure. It may be this through ACE inhibitory potential (although this is admittedly weak) or benefitting the endothelium via nitric oxide related mechanisms (appears to be in better accordance with the amount of anthocyanins that reach the blood). Reductions in both diastolic and systolic blood pressure have been noted, and for the most part appear to be reliable in presence although not so much in magnitude of benefit (ie. blood pressure is reliably reduced, but the degree of reduction seems to vary).
In regards to diabetes and blood glucose control, Roselle appears to have limited evidence to support these claims but the evidence is so far in support. Mechanisms are not known, and the remarkable potency in animal studies seems to be markedly less in the limited human interventions looking at it. Roselle does appear to weakly inhibit carbohydrate absorption enzymes, yet is synergistic with Morus Alba (White Mulberry) in doing so; a tea made of White Mulberries and Roselle, although currently not supported in vivo, is possibly an effective carbohydrate absorption inhibitory tea.
The interactions of Roselle and weight loss are not too clear-cut, and it seems to be highly intertwined with studies on Roselle toxicity; Roselle is known to be toxic in higher doses, and weight loss more often than not precedes chronic toxicity. For studies that note weight loss without toxicity, it seems to be related to reduced food intake in rats and mice rather than direct fat burning effects.
The appetite suppressing effects seem to be fairly reliable in rats, but caution should be taken in applying these effects to humans. Aside from not being reported as a side-effect in any human study, the bioactive known as Hibiscus Acid is similarly structured to (-)-Hydroxycitric acid from Garcinia Cambogia which is known to reduce appetite in rats reliably but not humans.
Low doses of Roselle tea or supplements appear to be effective in reducing blood pressure, and may be anti-diabetic. It is unlikely that Roselle can cause weight loss independent of a reduction of appetite
The toxicity itself seems to occur in mice and rats in a similar idea as the blood pressure reducing effects in humans, as in they occur reliably although the dose required to induce toxicity and what exactly occurs seems to vary from one study to another. This may be related to the exact molecules mediating toxicity not being known right now. For the majority of toxic effects, the lowest they have occurred is 200mg/kg in rats (2.2g dried calyx for a 150lb human). Human studies have used this dose or above with no apparent side effects though. The toxicity of these doses of Roselle need to be evaluated more.
One concern that does exist is testicular toxicity, which occurs fairly reliably at 200mg/kg or above in animals but has not been investigated in humans. Roselle appears to be anti-fertility in men, inducing abnormal sperm morphology. In females, there was a series of studies suggesting Roselle could cause abnormal (higher) birth weights in offspring with a delay of pubertal onset; for the most part these are attributed to the appetite suppressing effect causing maternal malnutrition, with no per se mechanisms harming the pup (via lactation) currently known.
Although these toxic effects can possibly be avoided by adhering to proper dosing, the amount of safety information in humans is not as expansive as would be desired; the therapeutic threshold (degree of 'safety buffer' between the active dose and toxic dose) is also lower than desirable, so possible toxic effects with overdosing Roselle is probably more relevant than other supplements.
Higher doses of Roselle do exert toxic effects, although none of these toxic effects have been reported in humans (that being said, they have not conclusively been disproven either). It would be prudent to avoid taking too much Roselle, especially since many benefits of Roselle (elaborating on in complete summary) are not dose-dependent above the lowest observable toxic dose of 2.2g/150lb human