Close equivalent can still be something you can't take. For example, if you're one of the unfortunate people with no activity on your CYP2D6 enzyme, there are e

Close equivalent can still be something you can't take. For example, if you're one of the unfortunate people with no activity on your CYP2D6 enzyme, there are essentially 4 pain medications that you can take at appropriate doses once you need something stronger than advil: Morphine, Dilaudid, Oxymorphone and Fentanyl, and there are reasons why doctors prefer other narcotics before prescribing any of those 4. Other pain medications will work, but you'll likely need inappropriately higher doses[0], which can lead to other problems (like overdosing). Likewise if you process faster than a normal person (i.e. you have more activity on that enzyme) you can overdose at normal prescribing doses. Close equivalents for some people can indeed be the difference between life and death, even if the condition itself isn't life threatening. [0]: https://www.ncbi.nlm.nih.gov/books/NBK100662/ https://www.ncbi.nlm.nih.gov/books/NBK100662/