Androgenetic alopecia, which you can inherit from either your mother's or your father's side of the family, actually works through the mechanism of a hormone called dihydrotestosterone (DHT). DHT is a derivative of testosterone, and is normally present in hair follicles and helps regulate the growth of hair. As men grow older, hormonal changes cause the amount of DTH present in hair follicles to increase, and those with hair follicles more sensitive to DHT notice that they are starting to lose hair.
DTH has this effect by working on hair follicles to shorten the growing phase of the hair. Normally, each hair lives 4-6 years, and its lifespan is divided into a growing phase (catagen), resting phase (anagen), and falling phase (telogen). As DHT works on hair follicles, each successive hair which grows out of it is finer and lives less long than the previous hair: that is, DHT shortens the catagen and anagen phases of hair growth. Eventually, the hair growing out of the follicle is so fine and short it is invisible, and then hair stops growing completely and the follicle dies. When this happens to a large number of hair follicles in your scalp, you appear bald.
A number of drugs you might be taking for other conditions might stimulate the production of DHT and make your hair fall out. For men with MPB, a particular class of drugs for hypertension (high blood pressure), called beta-blockers, often accelerate hair loss.
Drugs for gout, heart disease, depression, weight loss, attention deficit disorders, acne, epilepsy, fungal infections, hormone replacement therapy (HRT), birth control, ulcers and thyroid disorders can also cause hair loss. Remember, while some of these medicines have this side effect, not all the medications will. Also, your hair loss may be caused by other factors even if you are taking any of these drugs. But if you are under treatment for any of these conditions, check with your doctor or your pharmacist if your particular medication has this side effect. Shifting to another drug might help if the hair loss is only caused by the medication.
Drugs which are prescribed for hair loss, like finasteride (brand name: propecia), often work by blocking formation of the DHT hormone.