Cannabis is the only caveat there because the nics check is federal, and medical cannabis programs are all at the state level. Cannabis is still illegal at the federal level, so the feds don’t distinguish who is a “legal” cannabis user and who is not. They do not even distinguish who is an “addict” and who is not, when it comes to cannabis.

In fact, if you do leave that box unchecked and you are a medical patient, or fail a drug test administered during some other federal legal issue, that could be considered “proof” of lying on the form, and you could be charged.