by that definition, but fentanyl and
buprenorphine are not. It does have activity at the mu opiate receptor, but the activity is very different from that of agonists like methadone, oxycodone, or fentanyl. Buprenorphine is a partial agonist, which causes tolerance that is static, rather than the ever-increasing tolerance caused by agonists. It also has a 'ceiling' to it's agonist effect.