(HealthDay News)  The combination of phenylephrine and acetaminophen results in a pharmacokinetic interaction triggering increased plasma phenylephrine levels, according to a letter to the editor published in the March 20 issue of the New England Journal of Medicine.

Noting that phenylephrine is commonly substituted for pseudoephedrine in treatments for the common cold, without additional safety or efficacy studies, Hartley C. Atkinson, M.Pharm, Ph.D., from AFT Pharmaceuticals in Takapuna, New Zealand, and colleagues discuss a pharmacokinetic interaction between phenylephrine and acetaminophen.

The authors note that in three studies undertaken as part of the development of a new fixed-dose combination containing acetaminophen, ibuprofen, and phenylephrine, there was a pharmacokinetic interaction among the drugs. Administration of 10mg phenylephrine, 1,000mg acetaminophen, and 300mg ibuprofen resulted in a nearly four-fold increase in the maximal plasma concentration of phenylephrine (3,220pg/mL) compared with administration of 10mg phenylephrine alone (874pg/mL), and a two-fold increase in the area under the curve. A phenylephrine dose of 5mg produced a plasma concentration-time curve that was similar to that seen with administration of 10mg phenylephrine alone.

“These findings have implications from both regulatory and safety perspectives,” the authors write. “Multiple variants of acetaminophen combined with phenylephrine are now available on world-wide markets. Is further investigation required?”

Two of the authors are employed by AFT Pharmaceuticals.

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