。
I have a drug allergy.
Literal
Medicine [to-に] allergy [subj-が] exist-[polite-ます].
薬に is locative/target-に, not dative — 'an allergy exists with respect to medicine.' The pattern [allergen]に+アレルギーがある is the standard medical-intake template for declaring a drug allergy. アレルギー itself is borrowed from German 'Allergie' via Meiji- and early Shōwa-era medical vocabulary.