Drug Interactions Between Morphine and Orally or IV Administered Acetaminophen

Acetaminophen can significantly reduce the use of opioid analgesics when both are used concomitantly for treating moderate to severe pain. The use of IV acetaminophen used concomitantly with opioids has increased in practice for postsurgical pain relief over orally administered acetaminophen because it provides an immediate peak plasma concentration and is believed to provide a faster analgesic effect. Opioids used to treat pain inhibit gastrointestinal motility, including delaying gastric emptying. In patients receiving opioids the absorption of orally administered acetaminophen may be delayed and could result in gastric accumulation of acetaminophen thereby markedly changing the pharmacokinetic profile. The opioid-induced inhibition of gastrointestinal motility would not be expected to affect IV acetaminophen pharmacokinetics. Thus coadministered IV acetaminophen with opioid would yield better outcome in efficacy and reduced risk of side effects comparing with coadministration of oral acetaminophen and opioids.