Why is proving a case so difficult when the impairing drug of choice is marijuana? Police officers and prosecutors have become very skilled at investigating and convicting drivers impaired by alcohol. The triers of fact have been trained as to what signs and symptoms to expect in these cases. Alcohol is the “perfect drug” in that there is a defined amount at which all persons, regardless of tolerance, are impaired.

With the legalization of cannabis in many U.S. states, the accessibility of the drug by legal means has resulted in pervasive use. However, there is no longer the “number” judges are trained to expect. There is less predictability with standardized field sobriety tests and the expected results. There is no horizontal gaze nystagmus in many, if not all, cases. Essentially, most of the evidence upon which proof of alcohol impairment is based is gone.

There are three main factors that make marijuana cases tough. First is the public’s perception of the drug. The second is how cannabis affects the human body. The final consideration is how cannabis can be detected and linked to impairment, which is the key to a successful prosecution.

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