My experience with people on drugs is that their ability to make a correct judgement in any situation diminishes drastically.
For example, many of my friends would see cops everywhere after smoking weed (edit: secret cops, in civilian clothes. See how well they hide among us?). Another friend would always misplace her stuff, then start suspecting whoever was with her of stealing them (e.g. she would think I stole her baccy, though I don't smoke, etc).
A friend of mine who was a heroin addict (and died of it at the ripe old age of 34) used to tell me how heroine makes you sociable and friendly, but my experience was that when he shot, he would just fall asleep and then be too smashed to be anything like sociable. But in his head he was the soul of the party.
I've also known a fair few people who developed some kind of paranoid delusion most likely as a result of doing too many drugs. For example, I had a friend who thought one of his pals' brother was a hitman sent by the secret services to kill him. When he was sober, he was OK, most of the time, but when he was under- you couldn't make sense of him.
I mean, let's face it. You don't take drugs because they make you smarter.