The medical term for "male pattern baldness" is androgenetic allopecia. It is the most common type of hair loss and, colloquial name aside, it can affect both men and women. In men, androgenetic allopecia starts with balding on the crown of the head and makes an "M" pattern along the frontal hair line. In women, the hair loss is more generalized and diffuse, but the frontal hair line is usually spared.
Why does hair loss happen? The current theory describing the pathogenesis of androgenetic allopecia attributes hair loss to dihydrotestosterone receptors on hair follicles. When activated by dihydrotestosterone, these receptors cause the hair follicle to shrink and eventually the hair becomes too thin to pierce through the scalp, resulting in apparent hair loss. One enzyme involved in this pathogenesis is alpha-5-reductase, which converts testosterone and other androgens to dihydrotestosterone.
How do we treat androgenetic allopecia? Two of the most popular treatments, that are supported by research and do not involve surgery, are using rogaine (topical minoxidil) and propecia (oral finasteride).
Minoxidil is nice because it is a topical solution, meaning you spread it over your scalp and very little of it gets absorbed by the rest of your body. Unfortunately, we aren't exactly sure how minoxidil actually works; however, studies do support that it does actually restore hair growth in both men and women. Minoxidil's most common side effect is causing a contact dermatitis.
Finasteride is taken in pill form and works by inhibiting alpha-5-reductase, which reduces the amount of testosterone that is converted to dihydrotestosterone. Studies have shown that finasteride works at restoring hair growth in men and that it is significantly better than 2% concentrations of minoxidil; however, finasteride does not appear to help women with androgenetic allopecia. Rare, but concerning, side effects of finasteride include decreased libido and ejaculatory or erectile dysfunction. Health Canada recently released a statment about finasteride being associated with rare cases of male breast cancer. However, evidence suggests that all of these side effects are much more common in the 5mg formulation of finasteride, which is used to treat prostate disease. Androgenetic allopecia is usually treated with 1mg doses.
As with every treatment in medicine, there are obvious risks and side effects with any treatment, even for more cosmetic issues like androgenetic allopecia. We also do not have a lot of data regarding the long term affects for using the two therapies I discussed in this post. So make sure you take the time to assess how much risk you are willing to take to treat your hair loss before you begin taking either finasteride or minoxidil.
UpToDate has a great article on androgenetic allopecia, which was my major resource for this post.