。
Come back when you run out of medicine.
Literal
Medicine [subj-が] run-out-[cond-たら] come-[request-てください].
~たら is the conditional used for specific, one-time future occasions: 'when/if X happens (at some point), then...' Distinct from the general/habitual ~と conditional and the more abstract ~ば. なくなる ('come to not exist') is the intransitive pair of なくす ('lose, use up').