== More complete cross tolerance with methadone. == 

 I read that there is incomplete cross tolerance to certain opioids because they activate the mu-receptor by binding to different protein residues. Both fentanyl and methadone supposedly have structural elements that bind to distinct regions on the mu-opioid receptor that are the same apparently. This might be worth it to elaborate on in general (different binding sites on the Mu receptor itself) in another article and compile the opioids with similar affinities