吃药 is a separable verb (离合词) meaning 'to take medicine'. The two syllables 吃 and 药 can split apart when you need to insert resultative complements, aspect markers (了, 过), or duration/count phrases between them (e.g. 吃了药 'took medicine', 吃过一次药 'took medicine once'). Unlike English 'take medicine' where the object follows the verb, Chinese separates the verb and object. You cannot say 吃药了药 — instead, say 吃了药.