Anorexia. Effects include extreme weight loss, thin appearance, abnormal blood counts, fatigue, insomnia, dizziness or fainting, bluish discoloration of fingers, thinning brittle hair, soft hair covering the body, no menstruation, constipation, dry skin, cold intolerance, irregular heart beat, low blood pressure, ehydration, edema, osteoporosis.
Death is worst one. The most extreme "side effect" of anorexia nervosa is death, often from cardiac arrhythmia, heart failure, or kidney failure. Suicide is a frequent cause also. There is, of course, weight loss so extreme it damages vital organs such as the heart, kidneys, and brain. There can be calcium loss from bones, leading to osteoporosis, and loss of menstrual cycles. These are just a few.
In addition. It is important to know that anorexia is the psychiatric disorder with the highest mortality rate- about 5 per 1000. The probability of death increases as age of onset decreases (see arcelus et al. Archives of general psychiatry, 2011). Anorexia is a serious disorder that needs immediate treatment.