[Karen D]

Recent research has shown us that tramadol doesn’t work nearly as well in dogs as it does in people, because dogs lack the pain receptor that it targets. … It makes them sleepy while not actually making them less painful.
Tramadol is an opioid—essentially a synthetic version of morphine—and its painkilling effects in people depend largely on its conversion in the body to a substance called M1. But dogs don’t seem to convert tramadol to M1 as well as humans.