But it would only work on the people who take the drugs. This would be a huge advantage for the undrugged fighter!

I meant the medicinal drug....hopefully everyone intakes a pill within a month or two...

People already feel pain when they hit someone else, they just ignore it.

How are you going to get people to take the drug? You can't force them to against their will. And wouldn't it make people overly sensitive to pain in general? If you accidently stub your toe, would you be in agony? I don't think anybody would want to make themselves more pain sensitive, especially if they have jobs where they frequently suffer minor injuries.

Also, this wouldn't stop people from using wrestling or Ju Jitsu to fight.

The only medications I commonly take are pain reducers like aspirin or ibuprofen. If they put in an ingredient to increase pain, it would defeat the whole point of taking it in the first place. If you mean to use a drug that makes you literally feel the other persons pain, then please tell us when you invent it, as I'm sure you'll win a Nobel prize for inventing a telepathic drug!

sure, I'm going to take a pill to give myself pain...