Male Pattern Hair Loss (MPHL) is very common. About 50% of men 40 to 50 years of age are affected. Male Pattern Hair Loss describes a progressive thinning, or miniaturization, of scalp hair.

Male Pattern Hair Loss is sometimes referred to as Androgenic Alopecia or Androgenetic Alopecia.

The term Androgenic combines the words androgen and gene, because both androgens and a genetic predisposition are thought to play a role in MPHL. Androgens are hormones, such as testosterone. Miniaturization is a process by which hair becomes smaller and finer with each growth cycle until finally, it stops growing altogether. A progressive increase in dihydrotestosterone (DHT) is widely accepted as the cause of miniaturization. Men who are genetically susceptible to MPHL have increased levels of Type II 5 alpha-reductase, an enzyme that converts testosterone into DHT. Male Pattern Hair Loss can be halted by hair loss drugs such as Propecia (Finasteride).

The Norwood Scale can be used to categorize typical hair loss patterns in men. Typical hair loss is divided into 7 categories and sub-categories. Note that a Norwood 1 indicates no hair loss.