To understand hair loss, one should understand hair growth. There are three different phases of growth: anagen (growing phase), catagen (resting phase), and telogen (falling out phase). At any given time on a healthy scalp, approximately 90% of the strands is in the anagen phase, lasting anywhere from 2 to 8 years. The catagen phase lasts for about 2-3 weeks, during which the hair follicle decreases in size. Different types of hair loss can adjust these phases in different ways.
Medicines can play a factor in hair loss. Here you can find a list of medications that have been known to cause hair loss.
Which of these are you suffering from?