Marijuana DUIs are a hard thing to prove in a court of law, regardless of what law enforcement may try to tell you. There is no standard for marijuana impairment similar to a blood alcohol level. That's not to say that there aren't tests to determine if marijuana is in your system, because there are. However, there is no scientifically proven threshold which people can cross to go from 'I consumed cannabis' to 'I am too impaired to drive a vehicle safely because I consumed cannabis.'
That's why marijuana breathalyzer inventions fall short and are built on junk science. If they worked at all (which most don't), the breathalyzers would detect marijuana on your breath, but that doesn't mean that it detects impairment. My stepdad could smoke one bowl out of a pipe and be way too impaired to drive. On the other hand, my uncle can eat an entire tray of brownies, smoke like a broken chimney, and still perform any task involving reaction time and motor skills the same as he could prior to consuming any cannabis.
Let me be clear - I don't support impaired driving, whether it involves marijuana or anything else. If you have consumed too much over the counter meds, you shouldn't drive, just as a responsible marijuana user shouldn't drive after consuming too much marijuana. With that being said, I also don't think that people should be wrongly arrested for having marijuana in their system when they clearly weren't impaired at the time of their arrest for DUI.
The Arizona Supreme Court recently ruled that medical marijuana patients do not have immunity from DUI charges. Per The Cannabist:
The Arizona Supreme Court ruled Friday that medical marijuana cardholders don't have immunity from prosecution under a state DUI law that prohibits drivers from having in their bodies any amount of marijuana or its chemical compound that causes impairment.
But the state high court's unanimous decision Friday also said cardholders can try to show in court that they didn't have enough of the marijuana compound THC in their systems to be impaired. An attorney who filed a brief for a defense lawyers group said the ruling was welcome because it overturns a lower court's ruling that medical marijuana cardholders could be prosecuted for merely driving after using marijuana.
"Now at least they can drive," said David Euchner, an assistant Pima County public defender.
DUI charges can be a nightmare for any person arrested on suspicion of DUI. You get arrested, and automatically have your license suspended. The burden of proof is on the person that was arrested to prove that they were not impaired, which is not hard in court, but it still takes a tremendous amount of money to achieve that goal. If you are a marijuana consumer, medical or not, this is something that you should always fear. Never drive when you have consumed too much marijuana, and always be prepared to deal with a marijuana DUI because it could happen to you whether it's fair or not.