Fentanyl and Tramadol

Fentanyl and tramadol do both bind to the same receptors (mu), the difference is fentanyl is a complete agonist and tramadol is a partial agonist. This means you could use both of them together, but the fentanyl would be competing with the tramadol for receptors and more fentanyl would be required to achieve the desired response. at least that's my understanding of it.