On IV acetaminophen

 "Compared to oral acetaminophen, IV acetaminophen achieves a rapid elevation in plasma concentration and higher peak levels. The IV form achieves plasma levels rarely achieved by similar oral doses of acetaminophen and produces 75% higher central nervous system (CNS) bioavailability compared to the oral form. The analgesic effect peaks within one hour and lasts for four to six hours." 

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Darrell Harrington, MD 
Chief, Division of General Internal Medicine 
Harbor-UCLA Medical Center, Los Angeles 

 Source: Today's Hospitalist