Designer drugs are illegally manufactured modifications of prescription drugs, often stimulants.
Since these new chemicals were not named in the law, drug dealers caught possessing them couldn't be convicted. It didn't take long for the laws to be changed, but in the interim many other variations of the amphetamines were tried by humans with unexpected results. Even though they are chemically almost identical, the two drugs produce very different effects. This was not a pleasant experience for everybody. This resulted long-term psychotherapy sessions.