Is there a health concern with taking expired Vicodin?

Today has been a real bad back pain day. I always carry several ibufrofen with me along with an emergency Soma and an emergency 750ml Vicodin for such occasions. After the ibuprofen failed to do any thing I decided to take a Vicodin as I’m at work and the Soma tends to make me lethargic. This Vicodin had been in my little pill case for a long time. I noticed the size of it (horse pill) and realized this was from a real old prescription as the newer ones are slimmer. Anyway, I took it and twenty minutes or so later I’m dealing with the pain fine. This got me wondering about my old prescription pills. I have a ton of old Vicodin and I’m unsure if there is any danger in taking a painkiller that is long past its expiration date. Do these pills pose any threat to my health or do outdated medications just lose potentcy over time? And just how long past their expiration date are they effective? I’m hoping for a pharmacist’s or MD’s take on this.