Answered on Quora.
It probably isn’t.
Even very heavy marijuana use in adulthood does not appear to decrease IQ, and I believe IQ may be preserved in heavy alcohol use also and even alcoholism, believe it or not. It’s pretty hard to permanently reduce your IQ.
IQ can go up or down by up to 15 points in adolescence. No one knows why this is. But once you hit adulthood, it’s pretty stable. You can change a lot of things about yourself, but in general, IQ’s not one of them. Be happy with your IQ score. If it’s low, it’s not your fault and you did not make it that way. If it’s high, thank God for the probably inborn gift and realize that you did nothing to give yourself that high IQ. You just lucked out in the
Genetic Lottery is all. There’s no point in feeling pride about something that was simply given to you and you did nothing to earn.
A recent study showed that people who used cannabis heavily before age 18 did show IQ declines of up to 7 points. But this study is now being questioned. It’s good advice to wait until you are 18 to use marijuana on a regular basis. There seems to be a lot of problems with using marijuana in adolescence that are not present when you wait until adulthood to start.
A lot of times say with heavy drug use of different drugs, we can show mental declines in various areas on neuropsychological batteries. These are looking at specific subaspects of mental function. But even in these cases, IQ itself is often preserved.
IQ is pretty darn stable otherwise. It’s pretty hard to get a permanent reduction in something like that.