If you’re charged with drunk driving in Massachusetts and there’s evidence that you used drugs in addition to alcohol, prosecutors can usually present evidence of your drug use at trial.
The key case on this issue is Commonwealth v. Stathopoulos in which the SJC held:
A defendant may be found guilty of driving while under the influence of intoxicating liquor if the defendant’s ability to operate a vehicle safely is diminished, and alcohol is one contributing cause of the diminished ability. It is not necessary that alcohol be the sole or exclusive cause. It is enough if the defendant’s capacity to operate a motor vehicle is diminished because of alcohol, even though other, concurrent causes contribute to that diminished capacity.
This ruling is reflected in the model jury instructions for OUI liquor:
If the Commonwealth has proved beyond a reasonable doubt that the defendant’s ability to operate safely was diminished by alcohol, then they have violated the law even if some other factor tended to magnify the effect of the alcohol or contributed to their diminished capacity to operate a vehicle safely. Alcohol need not be the only exclusive cause. It is not a defense that there was a second contributing cause so long as alcohol was one of the causes of the defendant’s diminished capacity operate safely.
When a defendant is arrested and police suspect both drugs and alcohol contributed to his impairment, police typically charge the defendant with OUI liquor. Rarely do they charge OUI drugs (which is a separate crime). This may be due to the fact that OUI drug cases usually require the use of a drug recognition expert. For more, see my post OUI Drugs: Limits to Police Testimony at Trial.