daffyd,
I don't think it asks about illegal drug use. I think I'd remember, since the question might have involved me thinking back to my misspent youth - and I don't recall having to do that.
But anyway, amphetamines can be prescribed, or could at one point anyway. So, OK, yours weren't. But suppose you'd bought some over-the-counter antibiotics in a country where you could do that, would you call that illegal drug use? You didn't take heroin, or even cannabis. You took a prescription drug, which wasn't prescribed for you. Your choice, but it's debatable whether that constitutes illegal drug taking isn't it?
You seem to know me, and you say you're a regular prooner. Being idly curious, do you fancy sending me a pm saying who you are? No ulterior motive here, but no problem if you want to stay anonymous on anything relating to this subject.