Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2020 Publisher: Macarthys Laboratories Ltd, T/A Martindale Pharma, Bampton Road, Harold Hill, Romford, Essex, RM3 8UG, United Kingdom
Pharmacotherapeutic group: Nervous system, other nervous system drugs, drugs used in addictive disorders, methadone
ATC code: N07BC02
Methadone is a drug of addiction and repeated administration can result in dependence and tolerance. Cross tolerance with other opioids can occur.
It is a synthetic opioid analgesic similar to morphine although less sedative. It acts on the CNS system and smooth muscles via the peripheral nervous system.
The analgesic effect of methadone occurs about 10 to 20 minutes following parenteral administration. Miosis and respiratory depression can occur for more than 24 hours after a single dose. Methadone also reduces heart rate, systolic blood pressure and body temperature. Sedation is seen in some patients receiving repeated doses and sudden cessation of treatment can result in withdrawal symptoms.
Like morphine, it also has effects on bowel motility, biliary tone and secretion of pituitary hormones as well as on cough suppression. Methadone also causes the release of histamine from mast cells resulting in a number of allergic type reactions.
Methadone is rapidly absorbed following injection; however there are wide inter-individual variations.
Methadone is widely distributed in the tissues, diffuses across the placenta and is excreted in breast milk. It is extensively protein bound.
It is metabolised in the liver (forming inactive metabolites) and excreted via the bile and urine. Urinary excretion is pHdependent, the lower the pH the greater the clearance.
Methadone has a prolonged half-life (15 to 40 hours) and can accumulate on repeated administration.
No additional data of relevance to the prescriber.
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