You should discuss all use of non-prescribed drugs with your doctor on a routine basis.
However, in the case of the GP comment about Norway -- if there are legal ramifications, then I suppose this becomes a more complex issue to navigate. Though, if "socially unacceptable" drugs and their usages were not so stigmatized, this wouldn't be an issue either.
One can see something like this in the US a bit. There is a somewhat rare condition called CHS (Cannabinoid Hyperemesis Syndrome) where chronic, long-term users of cannabinoid products develop a condition similar to Cyclical Vomiting Syndrome. For ER professionals, telling the difference between the two (and other GI issues) can be extremely difficult -- especially if the patient does not disclose their usage of cannabinoid products. Some ERs are starting to drug test patients exhibiting symptoms of CHS/CVS to help in the diagnostic process, so they do not have to rely on patient honesty.
I agree, but the unfortunate fact is that it's not just Norway where you are one mistake away from being ratted out. General rules of thumb in this case should serve the individual. If you are very scared of drug interactions, you can just as easily find this information online.
There was a whole section of my application regarding cannabis usage. I provided my medical rec. There was a question about seeing a psychiatrist regarding cannabis usage (no). No questions about consumption method. My premium was that of the quoted non-tobacco premium, so I don't think it was affected at all.