Does leigheas mean both the field of medicine and a drug one could take?
Yes.
https://www.teanglann.ie/en/fgb/leigheas
I put in "he is studying" first time around and it was wrong ?
Irish and English both differentiate between the simple present tense (Déanann sé staidéar-"He studies") and the present progressive (Tá sé ag déanamh staidéir-"He is studying").
They aren't interchangeable.