Mostly birth defects. The birth defect risk is quite real; you cannot get pregnant while taking it. But with all respect to previous answerer, other dangers have been overblown, and no cause-effect has ever been proven for many of them. That doesn't mean they never happen, but in 30+ years of prescribing it I have never seen any of them. I agree it should only be prescribed by a dermatologist well trained in its use.
Very potent med with. Numerous potentially severe side effects: known to cause birth defects; can cause loss of bone mass, inflammatory bowel, liver damage, pancreatitis, psychosis, severe allergic reactions, hearing impairment, cataracts, inflamed optic nerve, & many others. Should be administered only by dermatologist well trained in its use & dangers.