“Nothing Exceeds Like Excess. You should know that Tony.”
-Elvira Hancock
In medicine, Paradoxical Drug Reaction is when the opposite outcome of a drug occurs rather than the expected outcome. An example (from personal experience) would be from relying on Benedryl to sleep and having it act (seemingly out of the blue) one day as an amphetamine.1
These sometimes occur when the user has been taking bigger doses due to increasing tolerance (having used Benedryl for seasonal allergies for years) and are common in certain types of drugs used too long. Any drug that effects your endocrine2 system (everything eventually does!) can trigger it.
Another perhaps more appropriate example is from steroid use. Take enough male steroidal hormones, and the body will eventually increase estrogen to rebalance itself. The more Testosterone they take, the bigger the feminine side-effects.
The steroid abuser will grow breasts, and their own methods for creating male hormones will shrivel up and die since they are no longer needed to do that work.