Your having had any "medical cannabis card" or not will not have any effect on or make any difference on your parents' being able to obtain visitors visas to the US. Your parents' applications will be approved or denied based on their own merits of proving "substantial ties" to their home country, their ability to support themselves while visiting the US, as well as their "intent" to return home after a temporary visit to the U.S.
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
If you are a US citizen, you could petition for them so they could immigrate to the US. The visitor's visa is simply a function of whether an immigrant will just come and visit and then return to live in their country. Give us a call to schedule a consultation to discuss your options. But I agree that your medical cannibis card has nothing to do with your parents' situation.
I agree with my collegues. Prescription medicine obtained lawfully does not affect your ability to petition for family members to get their lawful permanent resident status or their ability to get a tourist visa to visit you in the U.S.