The only caveat I would imagine with what you said is in the case of prescription drugs. Drugs of all kinds can have interactions when combined with together (both known and unknown), so disclosing the usage of non-prescribed supplements and alternative drugs can be vital information.
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.
