Most people are completely unaware that OUI drugs or OUI marijuana charges are very difficult to prove in court. There is no "litmus test" such as the breathalyzer results used in an OUI alcohol case.
OUI-marijuana in particular is hard to prove since the effects of marijuana typically wear off very quickly. We investigate immediately whether a booking video was made in your case. A booking video is virtually always an OUI-marijuana defendant's best friend. The effects of marijuana usually wear off in 25 minutes or so. Most people have sobered up considerably after they've been handcuffed, arrested, stuffed into the back of a police car, and finally reach the booking counter sometimes 30 minutes later.
When other drug use is alleged, the police must summon a drug recognition evaluator known as a DRE. They must conduct a series of 12 tests which take over 45 minutes. Most police officers are not certified drug recognition evaluators. They have to seek one from outside the department. Most of the time they are simply not available. Even when available, they often don't get there for two hours or more. By that time the effects of the drugs may very well have worn off. We have won many cases where our clients have been charged with OUI-marijuana or OUI-drugs.