Not really a big contradiction unless you want to be really pedantic. Over the counter painkillers and prescription painkillers are very different beasts. Painkillers as a social ill pretty much exclusively refers to the latter. Nobody would say “I’m on painkillers” while taking aspirin.
> Nobody would say “I’m on painkillers” while taking aspirin.
Really? That's exactly what one would say in my country... though they'd more likely to be talking about paracetamol. The strongest one that most people know about is Panadeine Forte (paracetamol + codeine).