The theory is that most hair loss drugs are designed to block the DHT at the androgen receptor level, and in doing so reduce the level of sensitivity to this hormone on the dermal papilla.
In turn this prevents shrinkage of the hair follicle and so to varying degrees, according to the drug used, it will prevent balding areas from appearing on the scalp. These drugs, largely used for hair restoration in a topical preparation, are known as DHT inhibitors.
One of the most popular of these is minoxidil which can be used in both male and female hair loss patients. It is unclear exactly how it works, and results can be very varied from person to person. It is used in topical form, and can either be prescribed, or bought over the counter branded as ‘Rogaine’. It does have recorded side effects with some people, causing dryness and itching in the area it is applied but this tends to be with higher strength solutions (5%), rather than the lower strength (2%).
Another drug which is reported to be effective in males only, is Finasteride (Propecia), which is more normally taken in tablet form as a prescribed medication. This works by preventing the testosterone from being converted to DHT, see what is DHT section. As with many other hair loss treatments it needs to be sustained for at least 3-6 months in order to have a good effect. However, there is at the moment considerable controversy surrounding the side effects of this drug because research is suggesting it may cause irreversible loss of sex drive and erectile dysfunction in some men.
An excellent alternative treatment to the above named drugs is the herbal remedy Saw Palmetto, used in combination with other herbal products, and research is indicating this can be more effective than prescribed medication.
Several other drug treatments, either used alone or combined with other therapies can help hair regrowth and these are all discussed in the article alopecia treatment which looks at the Cochrane review.