I believe that simply posessing a medical marijuana card should not be enough to pull someones carry permit. However, I do feel that there are certain drugs like alcohol, narcotics, and marijuana which if overindulged in can impair judgement to the point that carrying a firearm becomes a bad idea.
The libertarian in me says that carrying under the influence should not necessarily be a crime in an of itself. But I would not be opposed to adding it as a lesser included offense for someone who actually shot somebody under the influence.
One problem is that there simply is no legal standard. The legal limit for THC in the blood is zero. Similarly If you are found with opiates in your blood without a prescription the legal limit is zero. There is simply no legal structure or laboratory testing for levels of intoxication with substances other than alcohol. Common laboratory tests measure presence but not the amount of drug metabolites. Even if the amount could be measured the results would have limited meaning without knowing patient weight, metabolism etc.
Even in the case of alcohol blood levels only tell part of the story. I have treated patients with alcohol levels that would KILL me who are walking around talking and seem relatively sober. I have treated patients for DT's who were still well beyond the legal limit for intoxication.
As a result of this, I am SURE that any prosecuting attorney who found that a concealed carry permit holder was on any psychoactive medication may be likely to press charges. Equally I am sure that carrying under the influence would have plaintiff's attorneys foaming at the mouth to sue.
Does this apply to legal psychoactive medications? Possibly, if you were taking anti-depressants, mood stabilizers, anti-psychotics or stimulants (like Ritalin, or Adderall which are amphetamines) Those are sure to be factors in any criminal or civil case.
However, simply taking those medications is not, in many states, a legal reason to deny or revoke a concealed carry permit. It should be remembered that "mental illness" and "insanity" are two separate concepts, the former is a medical term and the latter purely a legal concept. One can be voluntarily undergoing treatment for mental illness and not be legally prohibited from owning or even carrying a firearm. Only when a court gets involved and mandates treatment do gun rights become an issue.