Yeah, it s a bad idea. It is UNLIKELY to affect the head although there s nothing out there one way or the other on safety. There is some small evidence of effects on learning being hurt by marijuana. Major brain deterioration... no, very little evidence of that. Here s the problems: 1) the big problem with concussion is that it predisposes someone to Really BAD effect if there are multiple ones in a short time. While it is NOT known if marijuana counts as further injury, waiting the time til it wouldn t be huge risk of another concussion (2 weeks after totally normal and NO headaches, confusion, etc.) would mean the risk is not increased from the usual use of marijuana. 2) people on marijuana do stupid stuff that sets up for antoher (worse, much much worse) concussion 3) it confuses the heck out of the issue. Let us assume you have a totally normal, indeed wonderful, experience with the next joint. Someone who knows about the previous concussion sees you a bit .. well... dopey. Reasonably they call the ambulance, you have to have a CT of the head to rule out subdural, thinking you don t want a thousand dollar hospital bill you start to run away, they figure it s a certain sign of a serious head injury, you get tasered and forced to have an evaluation, are stuck in the hospital for a required evaluation.......... Waiting a bit gets rid of all the issues.