Any prescription based drugs are practically impossible to CTB on OD. Certain barbiturates, opiates, opioids and salts (like Sodium Nitrite and Sodium Azide, but I do not recommend ever trying to use Sodium Azide) are the only things you can easily CTB by ODing on.
Even if there was a prescription based drug you could CTB on, you'd have to both consume an insane amount and deal with whatever terrible side effects they give you. I mean technically you can OD on paracetamol but I'd never recommend that, it'll take 2 weeks to kill you from liver failure and you'll be in terrible pain for those 2 weeks.