Reference

Glossary

Plain-English definitions of the technical terms used across SR17.org.

Last reviewed: 2026-05-05Editorial methodology

μ-opioid receptor (MOR): The brain receptor that morphine, fentanyl, oxycodone, methadone, buprenorphine, and SR-17018 all act on. The 'μ' is the Greek letter mu. Activation produces the characteristic effects (and risks) of opioids. Read more →
Agonist: A drug that activates a receptor when it binds to it. SR-17018 is a partial μ-opioid receptor agonist — it binds and activates, but only partially. Read more →
Antagonist: A drug that binds a receptor without activating it, blocking other drugs from activating it. Naloxone (Narcan) is a μ-opioid receptor antagonist — that's why it reverses opioid overdoses. Read more →
Biased agonism: When a drug activates one of a receptor's downstream signaling pathways more than another. SR-17018 was originally described as G-protein-biased — preferring the pathway thought to drive pain relief over the one thought to drive respiratory depression. Follow-up studies have complicated this picture. Read more →
G-protein signaling: One of the two main downstream cascades the μ-opioid receptor activates. In animal models, this pathway is associated with pain relief, sedation, and reward. Read more →
β-arrestin2 recruitment: The other main downstream cascade. Historically hypothesized to drive opioid side effects like respiratory depression, tolerance, and constipation — though this hypothesis is now contested. Read more →
Partial agonist: An agonist that activates a receptor but cannot produce the receptor's full maximal response, even at saturating doses. Buprenorphine is the classic clinical partial μ-opioid receptor agonist; SR-17018 is best understood as one as well. Read more →
Non-competitive agonist: An agonist whose effects are not reversed simply by adding more of a competing molecule at the same binding site. SR-17018 has non-competitive features — it stabilizes the receptor in an active state for an unusually long time after washout — but its effects are still reversed by orthosteric antagonists like naloxone. Read more →
Ki (binding affinity): A number describing how tightly a drug binds to a receptor. Lower numbers (in nanomolar, nM) mean tighter binding. SR-17018's Ki at MOR is ~11 nM (high affinity). Read more →
EC50: The concentration of drug required to produce half of its maximal effect. Lower EC50 means more potent. EC50 values for SR-17018 differ dramatically depending on which signaling pathway is being measured — that gap is the basis of its 'biased' characterization. Read more →
Emax: The maximum effect a drug can produce in a given assay, expressed as a percentage of the effect of a full agonist. SR-17018's Emax for β-arrestin2 recruitment is only about 10% — meaning it barely engages that pathway in standard assays. Read more →
Intrinsic efficacy: How strongly a drug activates the receptor it binds. A full agonist has high intrinsic efficacy; a partial agonist (like buprenorphine, or SR-17018 under most conditions) has lower intrinsic efficacy. Read more →
Therapeutic window: The ratio between the dose that causes serious harm (here, respiratory depression) and the dose that produces the therapeutic effect (here, analgesia). A wider window means a safer drug. SR-17018's window in mouse parenteral experiments is wider than morphine's; in oral experiments, the advantage shrinks. Read more →
Pharmacokinetics (PK): What the body does to a drug — absorption, distribution, metabolism, excretion. SR-17018's PK has only been characterized in animals; human PK is unknown. Read more →
Pharmacodynamics (PD): What the drug does to the body — what receptors it hits, what cascades fire, what physiological and behavioral effects result. Read more →
Respiratory depression: Slowed, shallow, or stopped breathing. The mechanism by which opioid overdoses kill. SR-17018 produces respiratory depression at higher doses, especially with oral administration. Read more →
Tolerance: When a drug's effect weakens over time, requiring higher doses for the same response. SR-17018 produces minimal tolerance in some pain assays (hot plate) but does produce tolerance in others (tail flick). Read more →
Dependence: A physical adaptation to a drug such that stopping it produces a withdrawal syndrome. SR-17018 produces dependence in mice. People using it can experience withdrawal-like symptoms on discontinuation. Read more →
PAWS (Post-acute withdrawal syndrome): The longer-tailed phase of opioid withdrawal that can persist for weeks to months after the acute symptoms resolve — typically insomnia, low mood, anhedonia, and intermittent cravings. Read more →
LC–MS/MS: Liquid chromatography coupled with tandem mass spectrometry. The gold-standard analytical technique for confirming the identity and quantity of a drug in a sample. Required to detect SR-17018, since standard immunoassays miss it. Read more →
Federal Analogue Act: A U.S. federal law (21 U.S.C. § 813) that allows prosecutors to treat structurally and pharmacologically similar non-scheduled compounds as Schedule I substances when they are intended for human consumption. SR-17018 is exposed to this statute. Read more →
Orphine: An informal name for a family of substituted piperidine benzimidazolone opioids. Brorphine is the most well-known illicit-market member; SR-17018 is the lab-research member that the family is now most discussed alongside. Read more →
Nitazene: A class of high-potency synthetic opioids that have appeared in the illicit supply. Mentioned on this site because they are a frequent dependence pattern that users describe substituting away from with SR-17018. Read more →
Naloxone (Narcan): The opioid antagonist used to reverse opioid overdoses. Available without prescription in all 50 U.S. states. It reverses SR-17018-mediated MOR activation, but repeat doses may be needed because SR-17018 can outlast a single naloxone dose. Read more →
Buprenorphine: A long-established partial μ-opioid receptor agonist (and κ-opioid receptor antagonist), FDA-approved for opioid use disorder and analgesia. The closest evidence-based analogue to what SR-17018 is being used for, and the comparator most often invoked. Read more →
Methadone: A long-acting full μ-opioid receptor agonist, FDA-approved for opioid use disorder. Decades of clinical data; provided through licensed treatment programs in the U.S. Read more →