Biotin, vitamin B7, is a water-soluble vitamin, also referred to as "vitamin H." In most people, biotin side effects are virtually non-existent, even for those who take extremely high doses of the vitamin. The most common side effect is cystic acne breakouts. Pregnant women, however, are at risk for miscarriage and should not take biotin.

A regulatory element necessary for gluconeogenesis, biotin is also required for fatty acid synthesis and amino acid production. Biotin aids in the body's metabolic functions, including regulating blood sugar levels and carbohydrate functions. It has an active role in the citric acid cycle and aids in aerobic respiration. It is also found in the amino acid, leucine.

The most common biotin side effects are breakouts of cystic acne at the jaw and chin line. The cause of this is unknown, and often subsides after a few weeks. Supplement takers also have reported a decrease in the acne if they keep their dosage to 2500 mcg or less. Each individual experiences a different frequency of acne.

Life-threatening biotin side effects are extremely rare. The only recorded case is of a woman who experienced a life-threatening lung and chest condition known as eosinophilic pleuropericardial effusion. She ingested a large amount of biotin along with vitamin B5. She recovered once she discontinued use of both vitamins. It is not known whether her condition stemmed from biotin, B5, or a combination of the two.

Biotin deficiencies are rare in normal, healthy adults. They have been seen in people who consume excessive amounts of avidin, found in raw eggs, or who have other existing skin and hair issues, such as phenylketonuria (PKU), a leading cause of dermatitis conditions. Deficiencies often take years to occur as biotin is naturally occurring and it is naturally recycled throughout the body's intestinal tract.

Studies have shown that as many as 50 percent of pregnant women are biotin deficient. They lack an enzyme that tells their body how to use biotin correctly, thus resulting in decreased metabolic functions. Most physicians do not recommend biotin supplements as treatment, however. Laboratory tests on pregnant rats have shown impaired fetal growth and miscarriage.

Biotin is often recommended as a supplement for healthy hair and nail growth. It is also used as a weight management supplement for its effect on carbohydrates. Biotin supplements are often intended for use for their reported cosmetic and metabolic benefits. Suggested dosages range from 3 mcg per day up to 5000 mcg per day for various purposes. Biotin side effects are rare enough and mild enough to manage, thereby making biotin a generally safe supplement.