服药 means 'to take medicine' and is a separable verb (离合词). The verb 服 and the object 药 can split when duration, frequency, or other elements need to go between them, as in 服了一次药 ('took medicine once'). It's the formal/written equivalent of the more colloquial 吃药 — you'll mostly see 服药 in medical instructions, prescription labels, and formal health advice.