Chemotherapy is the main cause of hair loss in teenagers with cancer. Chemo affects rapidly dividing cells like the cells in your mouth, your gut, your bone marrow and the roots of your hair. Not all chemo drugs cause hair loss and the amount of hair that falls out can really vary for different people. However, many people will lose most or all of the hair on their head. The hair becomes weak and begins to fall out. This usually starts around 2 weeks after the first lot of chemo. Some chemo drugs also cause other body hair to fall out like eyebrows, eyelashes, underarm and pubic hair.