Biotin is an essential vitamin that has been grouped with the B-complex vitamins since it was discovered, in yeast alongside other B vitamins. Although it is technically known as vitamin B7, this designation is not too common as it is usually simply referred to as biotin.
It was initially found to be a component of nails, skin, and hair to a relatively high degree. Biotin has been seen as the go-to vitamin for beauty ever since one pilot study in women with brittle nails showed supplementation to be beneficial. It is currently being marketed for improving nail, skin, and hair aesthetics. These claims, however, were not followed up scientifically so there is not much evidence to support biotin's role here. It can plausibly have these actions mechanistically, but there is simply not much evidence that can be used as support.
Beyond that, biotin's general role as an enzymatic cofactor has also led to some research suggesting it may interact with glucose metabolism in the human body. As a general statement it seems that in rodents with a higher circulating biotin level in their blood, the amount of insulin released in response to a glucose test is higher, leading to less elevation of glucose over time. This rodent evidence also suggests that the higher glucose is not met with higher insulin resistance suggesting a potentially beneficial role.
Not too much evidence has been conducted in humans in regard to diabetes, with one study finding that intramuscular biotin was able to attenuate symptoms of neuropathy in three diabetic subjects.
Overall, aside from instances where biotin may be deficient (alcoholism, some epileptic drug therapies, and overconsumption of raw egg whites) the supplement does not have any solid evidence for benefits and may have a role as a beauty supplement pending better evidence.
The only known supplemental dose of biotin that has been tested orally in humans, for the purposes of enhancing the quality of brittle nails, is 2.5mg taken once daily over six months.
This dose appears relatively safe although it is much higher than the recommended daily intake (RDI) of biotin which ranges from 25-30mcg (youth) upwards to 100mcg (adults). The biotin dose found in many multivitamins (30mcg or 0.03mg) seems more than sufficient.