The statistics don't make a distinction between the substances that caused the impairment. Stats Canada records the fact that there was a conviction for impaired driving. That's all Stats Canada has. The circumstances of the offence, whether there was a drug involved alone or together with alcohol is not.... Stats Canada doesn't have that information.
So you would include drugs as well. Whatever constitutes a conviction would be captured most likely, but there's no breakdown by Stats Canada as to what is drugs and what is not. There may be other studies that have looked at the prevalence of drug-impaired driving. I think there are some regional or city ones, but they're not national stats.