It’s definitely oversimplifying to say that it is stam unhelpful and stam illegal. My grandmother a”h was one of the first people in NY to get medical marijuana when in hospice care (just after it became legal) and it helped her immensely. It’s antinausea and somewhat mind-altering- really a blessing for someone who is on chemo and dying.
The question is whether R Genack would say that it’s a mitzvah if doctor-recommended in a state where it’s illegal. That’s interesting.
I’d say it may be mislabeling to kosher-certify it- but not because it’s illegal, because in some states it actually isn’t. Google New York State Medical Marijuana Program. If they can justify that it needs certification then they can definitely justify certifying it for medical reasons.