The contraindications are a VERY LONG list of things that can go wrong when you take a medicine. Every medicine that a doctor will give you has them. The contraindicators can be anything from a rash or headache, to death. The list is usually quite long with all medicines. This is the stuff that can happen that we do not want to happen.
Some side effects are rare, some are common.
Some are very mild and just a mere nuisance, some can put you in the ground. Medicines can be VERY dangerous. There is risk in all cures. Especially the farther one moves away from what is naturally in the earth. To illustrate what I mean, drugs and their side effects are listed below from THE PILL BOOK and The U.S. Pharmacopoeia.