At least they made mention of drugs available by prescription. Prescription drug abuse (whether by the intended patient or someone else) has long accounted for a large chunk of total substance abuse, a fact overlooked by some drug education campaigns even today. That may be because unlike with street drugs, such abuse doesn't generally attract the attention of law enforcement except in the case of forged prescriptions or medications sold on the black market.
Then of course there's over-the-counter medicine abuse. I don't know whether cough syrup (DXM) was commonly abused back in the seventies, but I understand it's very popular now, especially among teenagers. And as with certain Rx drugs, DXM abuse is potentially far more dangerous than marijuana has ever been.