Is using pot sinful for Catholics? In my opinion, it is. Grace builds on nature. It's undeniable that the general culture surrounding the use of pot is immoral, loose and destructive to the individual soul and society as a whole. Best to err on the side of caution and prudence. "Medicinal" pot use is the back door to legalizing pot for "recreation". Since, according to some, the church allows for the use of substances such as pot, this doesn't mean it must be allowed. It means in the strict sense it can be allowed. If the church allows the use of pot, then the use of it can be freely opposed. I oppose its use.
On the whole, it seems that none of the zealous "tolerators" of pot use smoke pot. Isn't that something. Some have jobs that test against its use. The military, civil service jobs test against the use of pot and none of the pot use "tolerators" can't/won't answer why this is. Try getting and keeping a CDL while using pot.
To LT and others there is no reason to second guess your convictions regarding pot use.