Alopecia, or hair loss, is a very common side effect of most forms of chemotherapy. Hair loss occurs as a result of the chemotherapy agents penetrating the hair follicle, causing the hair shaft to break at the root.
Hair loss may be temporary or permanent, and can have many causes. By far the most common type of hair loss is pattern baldness, also known as androgenetic alopecia. Men with pattern baldness initially lose hair in the front, crown, and sides of the hairline, and may become completely bald. Women are more likely to experience thinning at the crown and front of the head. This type of hair loss is hereditary, and is usually permanent. However, several treatments are available that may help regrow hair.

Causes

There are many possible causes of hair loss.

• Nearly all incidents of male and female pattern baldness  are due to family history.
• Major body stress. High fever, severe flu, surgery, and other stressful situations can cause hair loss, although it may not show up for weeks or months afterward.
• Some women experience hair loss several months after giving birth. The hair usually grows back.
• Recent high fever, severe flu or surgery. You may notice you have less hair three to four months after events such as an illness or surgery. These conditions cause hair to shift rapidly into a resting phase, meaning you'll see less new hair growth. A normal amount of hair typically will appear after the growth phase resumes.
• A number of drugs can cause sudden hair loss. Certain medications used to treat arthritis, depression, heart problems, high blood pressure, and gout may have this effect.
•  Hair loss may result from cancer treatments such as chemotherapy due to the toxicity of the drugs used. Your hair will begin to regrow after you complete your treatment.
• Systemic diseases often affect hair growth either selectively or by altering the skin of the scalp. One example is thyroid disorders. Hyperthyroidism causes hair to become thin and fine. Hypothyroidism thickens both hair and skin.

Treatment

If hair loss is caused by a temporary situation such as medication, stress or insufficient iron, the hair loss will stop when its cause ends. Hair loss due to infection may require oral antibiotics.
Alopecia areata can be treated with injections of steroids such as triamcinolone into the area. For all of the causes, early treatment works the best.
For hair loss caused by androgenic alopecia, there is no cure, although many treatments are available. You can purchase the FDA approved hair loss treatment minoxidil without a doctor's prescription, it's rubbed directly onto the scalp and works for both men and women. Finasteride (Propecia) is a prescription hair loss treatment that is prescribed for men only.  It usually works best for hair loss at top and back of the scalp, where there is still some remaining hair. This is available over the counter at most pharmacies.