Four of the 16 articles had 3 arms in the study: a placebo group, an IV acetaminophen group, and an active comparator or a second IV acetaminophen group with different timing of dosing. One article had 4 arms, including an active comparator. Therefore, a study is a single comparison and one article may contain more than one study. A total of 22 study comparisons were analyzed: IV acetaminophen to an active comparator (n = 8 studies) and IV acetaminophen to placebo (n = 14 studies). Rescue analgesic was morphine (n = 9 studies), meperidine (n = 5), oxycodone (n = 3), ibuprofen (n = 3), and tilidine (n = 2), which is an opioid only available in Europe.