There are many ways of expressing your topic, and you should try several of them. Some will work better in some databases than others, depending on whether the content focuses on medicine, law, or philosophy. Always look through your results for ideas on other terms to try.
Possible keywords related to "Medical Assistance in Dying" are:
It's best to use quote around a specific phrase, so it only finds items where those terms are together and have a special meaning.
You can sometimes use an Advanced Search and enter terms in different search boxes, joined with AND.
"physician assisted" or "medical assistance"
dying or death or suicide
Play with it; it's a creative process, not a prescribed one.