I've long been aware of studies suggesting that some prescription medications can retain their potency for several years past the expiration date shown on their labels (which is typically one year after they were issued to a customer). I must admit, though, I've recently been pleasantly surprised by one prescription issued to me twenty years ago.
I recently had a bout with severe, immobilizing back pain, which is slowly easing off (the inevitable result of a partially disabling injury back in 2004, which resulted in a spinal fusion and permanent nerve damage). The doctor (not my usual one) initially prescribed a couple of days' worth of Tylenol 3, and advised me to get it extended by my primary care practitioner when I got home.
Unfortunately, I couldn't get in to see her in the short term; so I looked in my "stash" of previous prescription medications to see what I had that might suit. I found a prescription for Vicodin issued to me in May 1999. I wasn't sure whether the remaining pills would still be any good after 20 years, but I had nothing to lose, so I tried one. Its potency was probably less than when it was made, two decades before, but it was still more than strong enough to be effective. It's providing as much pain relief as Tylenol 3, from a lower daily dose (1 tablet every 4 hours, versus 2 Tylenol 3 pills every 6 hours). I've got enough to last several days.
I note that the US military saves billions in drug costs by extending its "safe use" period for many medications.
The American Medical Association (AMA) concluded in 2001 that the actual shelf life of some products is longer than the labeled expiration date. The AMA stated the best evidence resides in the Shelf Life Extension Program (SLEP) undertaken by the FDA for the Department of Defense.
The original purpose of the SLEP program was twofold: to determine the actual shelf life of stockpiled military medications for future use, and to save government dollars. Over 3000 lots, representing 122 different drug products, were assessed in the SLEP program. Based on stability data, expiration dates on 88% of the lots were extended beyond their original expiration date for an average of 66 months. Of these 2652 lots, only 18% were terminated due to failure.
. . .
These results suggest that many drug products may have extended shelf lives beyond their expiration date. However, it is difficult for any one consumer or health care provider to know which product could have an extended shelf life. The ability for a drug to have an extended shelf life would be dependent upon the actual drug ingredients, presence of preservatives, temperature fluctuations, light, humidity, and other storage conditions. Additionally, the drug lots tested in the SLEP program were kept in their original packaging. Once a drug is repackaged into another container, as often happens in the pharmacy, the shelf-life could decline due to environmental variations.
. . .
Solid dosage forms, such as tablets and capsules, appear to be most stable past their expiration date. Drugs that exist in solution or as a reconstituted suspension, and that require refrigeration (such as amoxicillin suspension), may not have the required potency if used when outdated. Loss of potency can be a major health concern, especially when treating an infection with an antibiotic.
There's more at the link.
Since reading about that study, soon after its completion, I've made a point of keeping old prescription drugs when I had some left over, with particular emphasis on pain medication after my 2004 injury (which has left me in pain 24/7/365). As the authorities have cracked down more and more on the issue of effective pain medication, I've been able to draw on my stash for bad pain days, using current, less effective prescriptions on the days when the pain level is merely routine. It's been a blessing.
Provided you can store your old prescriptions in a stable environment, and secure them so that kids and/or potential abusers can't get at them, I see no reason not to keep them for future use when needed. In an emergency, if you can't get to a doctor or pharmacy for some reason, they might be really useful. (In rural and bush Africa, where it's frequently very difficult to get hold of some medications, I've seen individuals trade them to each other in emergencies, and done so myself on occasion. "I'll swap you twenty Doxy and a pack of cigarettes for thirty Cipro" - literally. Where there's a need, there's a way.)
I think the warning above about loss of potency is certainly something to keep in mind, particularly for antibiotics, as a drug that's too weak may cause your illness or infection to be prolonged, or even get worse. Other than that, the only difficulty I can think of is drug testing. If you don't have a current prescription for the drug(s) concerned, and your employer demands frequent drug tests, you may find questions being asked if they're detected in your system. Fortunately, that's not an issue for me.