Diphenhydramine
"A little known and rare side effect of this drug is a condition that would probably be called idiopathic thrombocytopenic purpura (ITP). This describes a serious depletion of platelets in the blood. The blood will not clot properly and an injury can be life threatening due to uncontrolled bleeding. It's rare but if it happens and the cause is not figured out by your physician(s) this is a big problem."