Men and women can lose their hair for a number of different reasons. Ignoring the side-effects from some medications and treatments such a radio- and chemotherapy, the most common cause is that hair follicles become less active as people age. More than half of all men are affected to some degree after the age of 50 years. However, a number of men have a genetic predisposition to begin losing their hair from the age of 20 years onwards. In all cases, hair slowly falls out and is not replaced by new growth. There is also a slow process of thinning. Every time a hair grows from a follicle, it is slightly thinner than before. Over the decades, new growth can become shorter and less visible in thickness and colour. This is particularly common in women as they age.

In men, there is a characteristic loss of hair from the head producing a fairly consistent M-shaped pattern rising from the temples. The condition is genetic and called androgenic alopecia. For these men, prevention is the only possible treatment. There is no product on the market that will stimulate the regrowth of hair from roots that have already gone dormant, although there are some possible medicines going through early development trials which may prove effective. In clinical trials, Propecia is shown to be the most effective medication for preventing hair loss on the head. It has no effect on body hair.

Propecia does produce some regrowth on the crown, less on the hairline on the front of the scalp. Thus, the earlier men start to use this medication, the better the long term results. But, if this is not male pattern baldness but loss from different causes, e.g. fungal infection, propecia will be completely ineffective. Other remedies will have to be tried.

There is clear evidence that propecia can cause damage to a foetus. This medication should therefore not be taken by any woman who is planning to conceive. For a time, doctors refused to prescribe propecia to all women. This precautionary approach was reinforced by a single clinical study which found that it did not slow hair loss in postmenopausal women. However, there are a number of reasons why this one clinical trial may not have produced a reliable result. Thus, some doctors do now prescribe Propecia to younger women who cannot conceive or who are taking adequate contraceptive measures.