Buprenorphine/naloxone is a combination of buprenorphine (a semi-synthetic partial opiate agonist) and naloxone (a full opiate antagonist). It is used in opioid substitution treatment as an alternative to methadone.
Both buprenorphine and naloxone are rapidly metabolized in the small intestine and liver and both have very poor bioavailability when taken this way. For this reason buprenorphine/naloxone is taken sublingually: buprenorphine has rapid absorption and good bioavailability via this route, whereas bioavailability for sublingual naloxone remains poor. Once absorbed, buprenorphine undergoes rapid distribution and readily crosses the blood-brain barrier. Although the serum half-life of buprenorphine is only around 3 hours, it has a very high affinity for the mu-opioid receptor, and once across the blood-brain barrier it will exert a clinical effect for 24-36 hours.
The naloxone is to discourage intravenous use. If taken intravenously, naloxone has good bioavailability and exerts a strong antagonist effect at the opioid receptors. This is likely to cause unwanted symptoms of opiate withdrawal in those who are opiate tolerant.
As a partial opiate agonist, buprenorphine exhibits a plateau effect with increasing dose and this makes it far safer in overdose than a full agonist such as methadone. As the risk of accumulating a toxic dose is reduced, doses can be increased more frequently during the induction/ titration phase of treatment, and with specialist drug service supervision, daily increases in dose are possible until stability is achieved.
*See Appendix 3 Pharmacology and Pharmacokinetics of Methadone and Buprenorphine page 97 of " New Zealand Practice Guidelines for Opioid Substitution Treatment 2014"
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