Drugs usage impair your mental facilities, reduce your physical ability, leads to addiction and social problems, causes disease(s), and so on and so forth...

Depends on the drug; Is cigarette/nicotine/tobacco included? What 'side effects' do you propose to be eliminated from these drugs? Wouldn't eliminating the side effects cause the drugs to lose their potency and making them less 'fun'?

I have tried several, some I got addicted to, some I can't stomach (marijuana -blerghh), all in the interest of science too btw...

IIRC:
Recreational drugs: Alcohol, tobacco, ganja, ecstasy, meth, LSD (old school), heroin (one of the most dangerous around -addictive and destructive), pil kuda (wth), cocaine
Medical drugs: Cough/cold medicine -most abused & used as replacement for hardcore addicts, morphine -for cancer patients, right?
Performance enhancers: steroids and...?

If that's the case then we're not talking about recreational drugs anymore right?

Historically drugs 'usage' are prevalent in practically every culture/civilization -older Malays always talk about how their grandparents work in paddy fields all day with nothing more than a roll or two of ganja cigarettes (and a lot of rice), the American Indians use tobacco as peace offering in tribal meetings, Western Europeans with their beers and seamen (and pirates?) and their rums, all these people do contribute to the society one way or another and it seems the drug habit kinda helps -except for American Indians, I guess...

Fast forward to modern times we see drugs usage as taboo -so that's one of the biggest obstacles you have to face, how to market a 'safe' drug that benefits healthy, normal people.

Secondly, scientific: Don't you think that if all these pharmaceutical/chemical behemoths can develop these super drugs they would have done it already? Though I guess we can see some instances of these super drugs abuse in sports for example.

Thirdly, ethical: We are conditioned to believe that our achievements must be gained through effort and hard work, using artificial help is cheating/corrupt practice. And how are you gonna stand up to the various anti-drug lobbies and pressure groups?

Lastly, super drug proposal/brainstorm
For supermen: A combination of alcohol (remove inhibition), morphine (painkiller), steroid (speeds muscle recovery) and adrenaline; remove drowsiness, hallucination, addiction, and heart condition side-effects; probably kill a few hundred monkeys and lab rats and cause the first few human test subjects to explode...