This is an important concept. It is why buprenorphine is unique as a treatment medication.
An agonist is a drug that activates certain receptors in the brain. Full agonist opioids activate the opioid receptors in the brain fully resulting in the full opioid effect. Examples of full agonists are heroin, oxycodone, methadone, hydrocodone, morphine, opium and others.
An antagonist is a drug that blocks opioids by attaching to the opioid receptors without activating them. Antagonists cause no opioid effect and block full agonist opioids. Examples are naltrexone and naloxone. Naloxone is sometimes used to reverse a heroin overdose.
Buprenorphine is an opioid partial agonist. This means that, although buprenorphine is an opioid, and thus can produce typical opioid effects and side effects such as euphoria and respiratory depression, its maximal effects are less than those of full agonists like heroin and methadone. At low doses buprenorphine produces sufficient agonist effect to enable opioid-addicted individuals to stop the misuse of opioids without experiencing withdrawal symptoms. The agonist effects of buprenorphine increase with increasing doses of the drug until it reaches a plateau and no longer continues to increase with further increases in dosage. This is called the “ceiling effect.” Thus, buprenorphine carries a lower risk of abuse, addiction, and side effects compared to full opioid agonists. In fact, buprenorphine can actually block the effects of full opioid agonists and can precipitate withdrawal symptoms if administered to an opioid-addicted individual while a full agonist is in the bloodstream. This is the result of the high affinity buprenorphine has on the opioid receptors. The affinity refers to the strength of attraction and likelihood of a substance to bind with the opioid receptors. Buprenorphine has a higher affinity than other opioids and as such will compete for the receptor and win. It will “knock off” other opioids and occupy that receptor blocking other opioids from attaching to it. If you try to take opioids on top of buprenorphine, you will experience no effect.