In pharmacology the term agonist-antagonist or mixed agonist/antagonist is used to refer to a drug which under some conditions behaves as an agonist (a substance that fully activates the receptor that it binds to) while under other conditions, behaves as an antagonist (a substance that binds to a receptor but does not activate and can block the activity of other agonists).
Types of mixed agonist/antagonist include receptor ligands that act as agonist for some receptor types and antagonist for others or agonist in some tissues while antagonist in others (also known as selective receptor modulators).
The best known agonist-antagonists are opioids. Examples of such opioids are:
- pentazocine, agonist at the kappa (κ) and sigma (σ) and has a weak antagonist action at the mu opioid receptor (μ).
- butorphanol, partial agonist at μ- and a pure agonist at κ-opioid receptor and antagonist activity at the delta opioid receptor (δ)
- nalbuphine, κ-agonist/μ-antagonist analgesic
Agonist–antagonist opioids usually have a ceiling effect – over particular dose they don't increase their potency. Hence agonist–antagonist opioids have a lower addiction potential but also lower analgesic efficacy and are more likely to produce psychotomimetic effects.
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