Studies have shown many dry meds are fully potent (defined as >90%) even 40 years after manufacture. The "3 years" expiration so common on stuff is nearly always an invented number. I'd treat the dry caber as potent but you can always get prolactin bloodwork to confirm.
Real expiration times tend to be much more obvious and don't fit some suspiciously convenient 1, 3, or 5 year pattern. E.g. HCG packaging will claim 30 or 60 days duration post-reconstitution, and that's refrigerated. Insulin, HGH, tetracycline are more examples.