医学 「いがく」 seems to be the field of medicine
医術 「いじゅつ」 might be another word for the field of medicine?
薬 「くすり」 is the word I found listed most often.
However, I also saw 医薬 「いやく」 and 薬剤 「やくざい」 but don't know what the differences between these words are.
No comparison; Go with 「薬{くすり}」 or 「お薬」 as a patient or someone on the patient's side. It is hard to beat kun-yomi words in intuitiveness.
「医薬」 and 「薬剤」 are too technical/academic for everyday conversations unless you are a medical professional.
「医学」 and 「医術」 do not even mean 「薬」 in the first place. Those words refer to the study/field of Medicine.
