The liver also metabolizes prescription opioids. Once they enter, they undergo a process known as first-pass metabolism. This reduces the bioavailability of the drugs, and their potency. Once they are metabolized, they are removed from the body through the urine.
In comparison with the other painkillers, the metabolic pathway of prescription opioids is a bit more complicated. There are actually two of them.
The first relies on the cytochrome P450 enzyme. The other involves a process called glucuronidation. This pathway makes it easier for the drugs to bind to water. Both can be active at the same time, but the first one usually starts to work first.