Occasionally I’ve heard people use the word “drug” to mean the past tense of “drag”. How did this originate, where is it used, and is it common?
Marking the past tense of a verb by changing a vowel (a technique called ablaut, also used to distinguish words like “sing” and song") is as old as English. Verbs changed in that fashion are called strong verbs. Verbs whose past tense is marked by adding ed are called weak (or regular). I suspect that “drug” is older than “dragged,” though I’m having a little trouble pulling up a cite right now.
Wow. A true expert is in our midst. 
No, I’m a dilettante. At best I’m an auto-didact, and you know what they say about self-educated men.
I believe that while, in general, strong verbs are older, in this case, drug is a recent innovation in the style of sneaked/snuck and dived/dove which are also more recent aberrations than their -ed counterparts.
They have big feet?
They’re not worth the paper they’re written on?
[General Zod]
Why do y’all say these things, when you know I will kill y’all for it?
[/General Zod]
I wouldn’t be surprised to find I was wrong; I was getting. But I think both are as reasonable constructions as y’all.
They have a fool for a teacher!
Who’d have thunk “drug” was a verb.
Drug can be a verb. Consider
*Barry fell asleep at the party because he had been drugged.
*
Moreover, this sentence
I drug the sack of potatoes all the way to CBI headquarters, never suspecting it held the corpse of a murdered man
makes perfect sense. It doesn’t fit the most prescriptivist grammar, of course, but its form adheres to the basic form of our language; I don’t think anyone has any difficulty parsing it.
While I don’t use drug as the past tense of drag myself (except when making examples as above), I think calling it a huge mistake is wrong. It’s just a change in the language. The k in knight was once voiced, and now it is not; that doesn’t mean that the current pronuncation is wrong, just that the change happened long enough ago that we’re not generally aware of it.
Barry fell asleep at the party because he was drugged, so we drug him to drug rehab.
Yeh, still works.
I believe this is an example of over-generalization, where the conjugation of sing/sang/sung was mistakenly applied to “drag.”
I don’t particularly know why; just that all my linguistics resources offline and on take it as read that it’s newer. I haven’t found where they determine that. My assumption is that it simply doesn’t appear in the written corpus until after dragged was well established.
Is anyone calling it wrong, though? It’s extremely dialectical, I would say more so than either snuck or dove but I don’t think anyone is saying it’s wrong as such, which is pleasantly refresing.
Overgeneralization, yes, sing specifically, no or else we would expect drig/drag/drug.
Dope smugglers drug drugs across the border.
Did you drug on that joint?
Nope, just doesn’t track right.
Yes, but a lot of people get sing/sang/sung wrong the same way, as well as sink/sank/sunk.
And who’d think “drunk” was a verb?
Just don’t confuse hung with hanged.
Well I’ll be danged.
The drugged, drag queens were drug off to jail.