Switching from one drug to another

You can try:

"Drug Substitution"[Mesh] OR switch*[tiab] OR transfer*[tiab] OR transition*[tiab]

I also tried convert*[tiab] OR conversion*[tiab] OR swap*[tiab] for my search, but they didn't return any results after being combined with the drugs of interest.

taper*[tiab] OR induction*[tiab] OR initiat&[tiab] may be of use. The taper part applies to the drug which is being discontinued, and the induction/initiation part applies to the drug being started, of course - but this language is used in drug switching as well as monodrug therapy.

If the drugs are not drugs of abuse, then you could also try:
substitut*[tiab]

However, this search is NO GOOD if you're switching from, say, methadone to suboxone, because then you get all this stuff about opioid substitution, which is not helpful.