There are plenty of completely legal prescription drugs you aren't allowed to drive on. Having a prescription will certainly not exonerate you if you're smoking a joint while driving, same as if you were caught taking Xanax while driving.

Whether magistrates would look at the levels of THC in your blood and be sympathetic to an argument that you had consumed a long time ago (and thus were no longer under the influence) is another question. I tend to think magistrates would side with the law as written in most cases, if the law explicitly prohibits driving with any amount of THC detectable in blood tests.