。
The medicine is gradually losing its effect.
Literal
Medicine [gen-の] effectiveness [subj-が] gradually disappearing-[change-over-time-てくる]-[past-た].
~てくる paired with だんだん ('gradually') is idiomatic for describing changes that have unfolded up to now and are continuing. ~てくる orients the gradual change toward the speaker's perspective/present; the complementary ~ていく would orient it away into the future. なくなってきた = 'has come to be gone' = 'has gradually disappeared.'