It's a recreational drug. Unless a patient needs it to counter some other malady such as for pain relief, most doctors will say that less is better and none is best.
This is a thing physicians say but often don't heed themselves, and I don't think it singles out cannabis in particular.
The thing that horrifies me the most is physicians who smoke. There's an activity of which there is no safe level of doing other than "none", plus they've definitely seen what a smoker's lung looks like, and yet I've seen plenty of doctors who smoke regularly.
However, unlike anarchy, any harm to human life is very costly (because value of human life is infinite!), for example: killing of someone, suicide, death because of incompetence or laziness, or self damage because of self medication, etc. are «sins» for libertarians.