If Eddie was only using 20% of his brain before he began using the fictitious "superhuman" drug NZT, he is using even less after taking it. The drug compels him to live a risky and should I say outright brainless lifestyle: committing adultery with his landlord's wife (it is no wonder we find out later Eddie has an ex-wife, in addition to his present failed relationship with a girlfriend) seeking a loan shark for "financial services," forgetting to deal with said loan shark, going into the stock market as a day trader (which is essentially legalized gambling, with more plenty of smart losers for every smart winner) and betting "his" money on it for success, attracting a ton of attention in the process, wildly partying, and consuming other drugs on top of the experimental NZT... My guess if the movie were made in today's age, he'd be investing in crypto, since that is the latest financial gambling fad. Put simply, the drug turned a harmless nobody into a hyperactive pretentious yuppie.
The film does get some points though: it moves at a brisk place, includes decent performances from its cast, and does make an effort to show some of the negative consequences of performance-enhancing drugs.
Unfortunately, the ending leaves much to be desired, as Eddie is depicted to have suffered essentially no medical consequences - he is in fact shown retaining the positives of the drug, while having none of the negative side effects. The fictional NZT itself is some sort of miracle drug, almost as if it was concocted from the fantasies of real drug culture... The kind that, for example, claims drugs like cannabis can do everything from curing cancer, treating every mental illness under the sun, to "enhancing" one's intellect or creativity. While cannabis (marijuana) is an unfair comparison as it is not a performance-enhancing drug and doesn't tend to kill its users (being less dangerous than the likes of alcohol), I think you get the point.
FYI, odds are you already use 100% of your brain. The only 10% or 20% claim of brain use made in the film is fictitious. You are stuck with the brain you got, so treat yours well and don't take drugs.