5 Rare Old Medicines That Had Horrific Side Effects

4. Thymol.

Derived from the herb thyme, thymol is one drug that you may recognize as it is one of the constituents in the toothpaste product Euthymol. Side effects when ingested for the treatment of ringworm. Of course, there are the usual poisoning symptoms: nausea, vomiting, and headaches. More unpleasant side effects include a deep depression, paradoxical giddiness, eventual collapse, and possibly death.