With marijuana now legal for either recreational or medical use in the majority of states, drugged driving appears to be on the rise. Unfortunately, there is no straightforward way to test whether someone is actually impaired by marijuana. It's easy enough to test for the drug's metabolites, but those remain in the system for weeks, while the drug's "high" only lasts for a couple of hours. Law enforcement has been seeking a so-called 'pot breathalyzer" that could test for actual impairment, not just exposure to the drug.
Fighting a DUI involving breathalyzer results is never easy, but it is not impossible. Just like any measurement device, a breathalyzer has weaknesses and a general margin of error, so it is usually possible to build some sort of legal defense against such charges.