People can be quite emotional when it comes to their hair. The way we wear our hair broadcasts several things about us, including our general level of health and our age. Naturally, when people encounter what they consider to be hair problems like graying or balding, they react emotionally, and may be prone to accepting some of the myths about hair that never seem to die out.
It is easy to be tempted by shampoos that claim to make hair grow faster. There are several reasons why this is a fallacy. One is that hair grows at a pretty uniform rate: about 1 cm per month. Some shampoos can make hair appear thicker. They do this by swelling up each hair's cuticle, but they do nothing for the growth rate of hair. With few exceptions, human hair grows about 15 cm (6 inches) per year.
Also, once your hair has sprouted from a follicle on your head, it is dead protein. There is much you can do to take care of your hair and keep it as resilient as possible, but with the exception of the FDA approved drug minoxidil (in Rogaine), nothing you apply to your scalp will cause hair to grow. The shampoo Nizoral, which is available over-the-counter in the US and many other countries, is FDA approved for treating dandruff and seborrheic dermatitis of the scalp, but there is evidence that it has hair loss benefits, and if it doesn't in fact help new hair grow, it may work for maintaining the existing hair population. But until the FDA grants approval for this purpose, Nizoral is only officially sold as a treatment for dandruff.
Is there a supplement you can take that will cause your hair to grow thicker or faster? Basically, no. With several caveats in place, you can find exceptions. For example, if a person is malnourished, ill, or under tremendous stress, then taking vitamin and mineral supplements may help, because it is likely that such a person isn't eating properly and if the body has the choice of keeping its vital systems going or growing hair, it's going to keep the vital systems going.
For the average adult, there's not much compelling evidence that nutritional supplements make hair grow thicker or faster beyond anecdotal evidence. However, if you want to hedge your bets by taking a supplement, then why not? As long as you keep your expectations realistic.
Any hair growing tips that are meaningful have to do with overall lifestyle: good nutrition, exercise, avoidance of certain harmful substances, and good health habits. Healthy sources of protein (fish, soy, beans, yogurt) are good, as are vitamins B-6, biotin, folic acid, and inositol. Many advertised hair growing vitamins are simply variations on vitamin supplements you can get at your drugstore. Other types of hair growing pills should be taken with a healthy dose of skepticism.
Don't fall for the myths surrounding many of the hair growing products out there. Many of them sound too good to be true, and they are.