Dangerous weight loss as a result of an eating disorder can get in the way of a normal menstrual cycle. But when treatment works, girls gain weight and get their cycle back.
If you search hard enough, the Internet has what you're looking for. But people with anorexia may not have to look too hard to find videos promoting their eating disorder.
There are no medications approved to treat anorexia nervosa, but often patients are given psychotropic meds anyway. These meds have multiple side-effects and no clear benefit.
Eating disorders often go hand-in-hand with anxiety and isolation. So much so, that helping people with eating disorders learn to enjoy social interaction could improve the illness.
Anorexia and bulimia are serious medical conditions. Seeking treatment from a qualified and experienced authority can greatly improve outcomes.