Most police officers are not drug experts. This limits the type of testimony they can offer at trial in an OUI drugs case. The key ruling on the issue comes from Commonwealth v. Gerhardt.
In that case the Supreme Judicial Court writes:
We are asked whether a police officer may testify, without being qualified as an expert, to the effects of marijuana consumption and may offer an opinion that a defendant was intoxicated by marijuana. We conclude that an officer may not do so.
Additionally, a non-expert officer may not testify that a defendant’s performance on the field sobriety test established that he or she was under the influence of any particular drug.
Nevertheless, an arresting police officer may testify “concerning a defendant’s observable appearance, behavior, and demeanor.” This may include some of the common indicia of intoxication.
Whenever possible, police will use trained “drug recognition experts” to evaluate drivers who are suspected of being under the influence of drugs. Such an expert can usually testify at trial concerning what drugs likely caused the defendant’s alleged impairment.