Marijuana is a drug under Colorado law, even when you have a prescription for medical marijuana. There is no strict “legal limit” for DUI of marijuana in Colorado. But a jury may infer that you drove under the influence of marijuana if a blood test showed 5 nanograms or more of delta 9-tetrahydrocannabinol (THC) per milliliter in your blood when you drove. However, the fact that you possess a medical marijuana registry card does not, by itself, constitute probable cause for an officer to require you to submit to a Colorado DUI blood test for drugs if you are arrested. Nor can possessing a medical marijuana registry card be used as evidence to convict you of DUID or DWAI.