canada
See s. 246 of the Criminal Code:
Every one who, with intent to enable or assist himself or another person to commit an indictable offence, ... administers or causes to be administered to any person, or attempts to administer to any person, or causes or attempts to cause any person to take a stupefying or overpowering drug, matter or thing, ... is guilty of an indictable offence and liable to imprisonment for life.
This is a helpful standalone offence because it sidesteps the question of whether this step of administering the drug would have been considered a more-than-merely-preparatory act towards a sexual assault.
Section 246 allows the prosecution to charge and convict without having to prove that the step of administering the drug was more than a merely preparatory act towards sexual assault (that is what would be required to be proved to establish attempted sexual assault).
Under s. 246, all that needs to be proven is the intent to enable a further offence. This is in contrast to an attempt, which would require further establishing that the act was more than merely preparatory. This is a notoriously fuzzy line, something that would complicate a trial and introduce uncertainty of conviction.
For comparison with what would be needed if directly charging attempted sexual assault, see this other Q&A: For a criminal attempt, when is an act more than merely preparatory?
While tangential to this question, I see that the comments raise the common question about how to prove the requisite intent. It is proven like any other fact in issue: through evidence (including testimony and its cross-examination) and the judge or jury's weighing of all the evidence, including the credibility and reliability of any witnesses.