Propantheline Paracetamol

Nimmo I, Heading RC, Tothill P, Prescott LF. Pharmacological modification of gastric emptying Effects of propantheline and metclopromide on paracetamol absorption. Br Med J 1973 1 587-9. 

Since most drugs are largely absorbed in the upper part of the small intestine, drugs that alter the rate at which the stomach empties can affect absorption. Propantheline, for example, delays gastric emptying and reduces paracetamol (acetaminophen) absorption, (p.l92), whereas metoclopra-mide , (p.l91), has the opposite effect. However, the total amount of drug absorbed remains unaltered. Propantheline also increases the absorption of hydrochlorothiazide , (p.959). Drugs with antimuscarinic effects decrease the motility of the gut, thus the tricyclic antidepressants can increase the absorption of dicoumarol , (p.457), probably because they... 

Propantheline reduced the rate, but not the extent, of paracetamol absorption. This would be expected to reduce the rate of onset of analgesia. Other antimuscarinic drugs that delay gastric emptying would be expected to interact similarly. In one case, the diphenhydramine component of a paracetamol product delayed paracetamol absorption after an overdose, and complicated the evaluation of the risk of toxicity. 

Propantheline 30 mg intravenously delayed the peak serum levels of paracetamol 1.5 g in 6 convalescent patients from about 1 hour to 3 hours. Peak levels were lowered by about one-third, but the total amount of paracetamol absorbed was unchanged. This effect occurs because propantheline is an antimuscarinic drug that slows the rate at which the stomach empties, so that the rate of absorption in the gut is reduced. The practical consequence of this is likely to be that rapid pain relief with single doses of paracetamol may be delayed and reduced by antimuscarinics (see Table 18. r, (p.672), and Table 18.2 , (p.674) for a list) but this needs clinical confirmation. If the paracetamol is being taken in repeated doses over extended periods this seems unlikely to be an important interaction because the total amount absorbed is unchanged. 

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