Having once had mag sulfate, I would appoint that. Made every cell in my body hurt, set off all hurting receptors.
I had actually proposed that for truth serum, but in all honesty, I would have admitted to things I had Not done, in order to receive the pain to stop.
Capsaicin is actually an analgesic -- it stops pain. It just reacts with the mucous membranes to produce that burn )
Narcan is a narcotic antagonist -- and if you were in pain, being treated with opiates, it would negate their effects.
As for Causing pain directly -- to my knowledge, there is no med for this purpose, however numerous drugs have painful side effects.
Well, a drug is only a chemical compound, so whether you were to isolate the compounds in particular poisons or venoms, and administer those as a drug, then you could have some rather nastily hurting inducing drugs indeed.
It would also be possible to create a drug which activated rather than inhibited the neuro-transmitters associated with hurting. Thus you could create simulated hurting by fooling the brain into a neorological pain response.
Pain is only the brains reaction to a stimuli applied to a nerve. Now, whether we could understand the exact way in which this happens, it would be theoretically possible to create a drug which could, for example, give the symptoms of a stubbed toe.