biotin

While biotin (Vitamin B7) is the common answer, I'd like to point out the logic in how that recommendation comes about and go from there. It is well-documented that B7 deficiency in children causes fragile nails and hair loss, however, such symptoms have not really been found in adults. Also, the average European/American gets about 20 times the daily requirement of B7, so I highly doubt that is your issue. Hair and nails are made of proteins that your body has more difficulty fabricating as you advance in age, because protein resources are diverted elsewhere. Furthermore, high concentrations of other B vitamins can interfere with hair and nail growth, regardless of the concentration of B7 in your blood, because those vitamins will block B7 receptors in your cuticles and follicles. Damage from exposure to ultraviolet light and other harsh environments can impede or damage hair and nail growth. Lastly and most importantly, hormones from the body send signals to hair and nail beds in the skin to produce or stop producing hair and nails, and if those hormones fall out of proper balance, your hair and nails can become weak. In western culture, we are exposed to all sorts of stresses that interfere with hormone balance. The best thing you can do for your hair and nails is not to swallow $2 pills that contain a chemical your diet already supplies in abundance, it is to relax and stop stressing yourself out too much. (and also stay out of the sun for too long)