Another huge sin of the US drug war is mixing likely-to-be-abused painkillers like codeine with acetaminophen to lower their "abuse potential" -- but, with junkies not being rational, it leads to abusers destroying their livers.
The same is true of dextromethorphan (DXM), the main active ingredient in cough syrup.
For many naive teenagers, cough syrup is their introduction to recreational drug use because it's available OTC. Unfortunately, most teenagers are also impulsive and can't be bothered to do much research beforehand. Threads like this one below, titled "6,276 mg acetaminophen...am I gonna die?", are unfortunately very common.