No, you can't operate a forklift when using medical marijuana. It doesn't matter that your use of the marijuana would be legal; alcohol is legal, too, but you can't operate machinery while drinking or impaired from drinking. The problem with marijuana is, it is metabolized, or removed from the body, slowly compared to alcohol and you will test positive for it long after using it. It is also very difficult or impossible to determine how impaired you if there is marijuana in your system: there is no easy and reliable "blood-marijuana" test to show impairment, the way there are blood-alcohol (or breathelyzer) tests. So if there is any injury done by you when you operate the forklift and you test positive for marijuana, your employer would, as they note, be held liable for letting someone using marijuana operate heavy machinery or a vehicle. Hence they can and should refuse to let you operate the forklift if you have a medical marijuana card, since they are not required to expose themselves to that level of liability.And you should not want to operate a forklift with a medical marijuana card, since if anyone were injured, YOU could be personally sued, too, and again, if there is any marijuana in your system, would very likely be held liable. Some things you can't do while taking certain medications.