From what I've read it should still be fine for a few months after expiration. Also, with finasteride in particular, keep in mind that even doses of 0.25mg are still going to inhibit a lot of DHT. So even if it is expired and loses some potency, it's still likely only by a very small percent. Probably less than 10% of a reduction in potency. Nothing to worry about unless it's like a year old. Relax.
@DoctorHouse have you dealt with expired or near-expired finasteride? My last bottle was nearly expired too, but worked just fine.