Suxamethonium Cimetidine

One report suggests that recovery from the neuromuscular blocking effects of suxamethonium (succinyichoiine) is prolonged by ci-metidine, but this may possibiy have been due to the presence of metociopramide. Four other reports say that no interaction occurs between suxamethonium and either cimetidine, famotidine or ranitidine. Cimetidine, but not ranitidine, has been reported to increase the effects of vecuronium. Cimetidine does not aiter the effects of atracurium or rocuronium and ranitidine does not aiter the effects of atracurium. 

A study in 10 patients given cimetidine 300 mg orally at bedtime and another 300 mg 2 hours before anaesthesia, found that while the onset of action of suxamethonium (succinylcholine) 1.5 mg/kg intravenously was unchanged, when eompared with 10 eontrol patients, the time to recover 50% of the twitch height was prolonged 2 to 2.5-fold (from 8.6 to 20.3 minutes). One patient took 57 minutes to recover. His plasma cholinesterase levels were found to be normal. It was later reported that some patients were also taking metociopramide, which is known to interact in this way. See also Neuromuscular blockers + Metociopramide , p.l27. 

Not understood. Studies with human plasma failed to find any evidence that cimetidine in therapeutic concentrations inhibits the metabolism of suxamethonium. However, metoclopramide may do and therefore is possibly the drug responsible for any interaction seen. In vitro studies with very high cimetidine concentrations found inhibition of plasma cholinesterase (pseudocholinesterase) activity. The cimetidine/vecuro-nium interaction is not understood, but it has been suggested that cimetidine may reduce the hepatic metabolism of vecuronium. ... 

Information seems to be limited to the reports cited. The most likely explanation for the discord between the cimetidine/suxamethonium results is that in the one study reporting increased suxamethonium effects some of the patients were also given metoclopramide, which can inhibit plasma cholinesterase and prolong the effects of suxamethonium (see also Neuromuscular blockers + Metoclopramide , p.l27). In four other studies, cimetidine and other H2-receptor antagonists did not alter suxamethonium effects. Therefore, it seems unlikely that an interaction exists. There is some evidence that cimetidine may slightly prolong the effects of vecuronium, but ranitidine appears not to interact. Atracurium and rocuro-nium appear not to be affected. Overall these possible interactions seem to be of little clinical significance. 

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