I'm going with the fact that a drug addict implies that a person does hard drugs. Hard drugs are cocaine, heroin, fentanyl, meth, etc. If a person does pharmaceuticals, they are a pillhead. We do not call them a drug addict. If your mom is popping pills in the bathroom, she's a pillhead, not a drug addict. We don't want to conjure the image of her shooting up with a heroin needle when she is just popping pills. If a person only smokes a plant that contains THC, which is indeed a drug, we call them a pothead. This is just like if a person is addicted to caffeine, we don't call them a drug addict, even though caffeine is a drug. If a person is addicted to alcohol, we call them an alcoholic, even though alcohol, like THC, is a drug. I hope this clears things up for the dimwits in the peanut gallery.