There are clear impairment laws.  You can be charged for blowing over with cough syrup.  Labels and warnings say you can not drive or operate heavy machinery.  Of course your insurance company will take that into account during discovery and depositions.   If you are impaired, you are impaired, and will be dealt with accordingly.   If you have prescription marijuana or taking a prescription that impairs you, you should not be driving.  If you need to drive, find an alternative treatment.