They attenuate both emotional and sensory aspects of the pain experience. Strong agonists (ie, those with the highest analgesic efficacy, full agonists) include morphine, methadone, meperidine, fentanyl, levorphanol, and heroin. Drugs with mixed agonistantagonist actions (eg, buprenorphine) may antagonize the analgesic actions of full agonists and should not be used concomitantly. Codeine, hydrocodone, and oxycodone are partial agonists with mild to moderate analgesic efficacy. They are commonly available in combinations with acetaminophen and nonsteroidal anti-inflammatory drugs (NSAIDs). Propoxyphene, a very weak agonist drug, is also available combined with acetaminophen.