I had ACL surgery and my surgeon gave me a prescription for Percocet for the pain. It worked great but when I had the prescription refilled, I got Vicodin instead. The pharmacist assured me it would work the same. It didn't. When I went back and exchanged the Vicodin for Percocet, I got the pain relief I expected. The pharmacist gave me a funny look like she didn't believe me. Did I just make this up in my head?

Definitely not. It is well-known that different people respond to different drugs differently. This concept is referred to as patient variability in responsiveness. There are many reasons this happens and it can affect many different drugs not just narcotic pain relievers. Age, body fat and water content, sex (male versus female), and even race can contribute to ways in which the same drug is metabolized (broken down) differently. Drug metabolism determines (in part) how effective (sometimes how powerful) a drug can be. The reverse is true too. Undermetabolism and underabsorption of the medication results in no change (or minimal change) in symptoms. In the case of pain medications, some people need a higher dose of the same drug to get the same effect as someone who responds well to the lowest dose prescribed. And some people simply don't respond to the medication. In your case, even though the two drugs (Percocet and Vicodin) are very similar (they differ only in two atoms), the effect was very different. You may want to let your physician know about this and make it a part of your medical record. This can save you time, pain, and money in the future. Should you need similar pain control, starting with Percocet -- rather than trying Vicodin (or other drugs) first -- may be the fastest and most direct treatment approach.

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