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Monday 22 May 2006

Acetaminophen/Paracetamol Analgesia Blocked by Anti-Emetic Drugs

By: Medscape News

Results of a French study provide the first evidence in humans that co-administration of the anti-emetics tropisetron or granisetron with acetaminophen (also known as paracetamol) blocks the analgesic action of acetaminophen.

This is a potentially important observation, investigators say, given that the anti-emetics are frequently used concomitantly with acetaminophen in patients with cancer.

The interaction between these agents "needs to be evaluated in the clinic to determine whether it is of clinical importance," write Dr. Gisele Pickering from Centre Hospitalier Universitaire in Clermont-Ferrand, France and colleagues in the April issue of Clinical Pharmacology and Therapeutics.

Using an experimental electrical stimulus pain model in 26 volunteers, the team showed that the pain-relieving effect of acetaminophen was totally inhibited by co-administration of tropisetron or granisetron, two serotonin antagonists primarily used in the prevention of chemotherapy-induced nausea and vomiting.

These observations in humans support preclinical studies, which have suggested that acetaminophen "involves endogenous serotonin to exert its antinociceptive effects," the authors note.

Dr. Pickering's team also points out that the antagonism of acetaminophen analgesia by the anti-emetics was not a result of a pharmacokinetic interaction, because plasma levels were unchanged when tropisetron or granisetron was given concomitantly with acetaminophen.

Rather, the results favor a pharmacodynamic interaction between acetaminophen and tropisetron and granisetron - one that needs to be studied further, the authors conclude.

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