In a small Swiss study in the Journal of the American Academy of Dermatology in 1990, women with brittle nails who took biotin for several months had a 25% increase in nail thickness, reduced nail splitting, and improved nail structure, as seen under a special microscope. A few other old reports also hint at nail improvement with biotin, but they were small, too, and poorly designed and there have been no follow-up studies. Still, the idea of biotin therapy for brittle nails is not so far-fetched. It comes out of veterinary medicine, which uses high doses to correct hoof abnormalities in horses and pigs and nails are similar in composition to hooves.
According to the National Academy of Sciences, an "Adequate Intake" of biotin for adults is 30 micrograms a day. Biotin is found in a variety of foods, including soybeans, nuts, cereals, dairy, bananas, and sardines. Eggs are one of the best sources, though eating raw egg whites regularly can actually cause biotin deficiency. Multivitamins usually contain 30 to 300 micrograms (300 micrograms is the Daily Value, the recommended intake used on food and supplement labels). The amount commonly recommended for brittle nails is a lot higher (2,500 to 3,000 micrograms a day).
Keep in mind: Brittle nails are often caused by dryness from winter weather, hot water, and detergents, as well as over-manicuring, artificial nails, and other nail products. They can also just be a normal part of aging. But if you have very brittle nails, talk to your doctor, since they are sometimes associated with fungal infections or a medical condition such as thyroid disease.