(J Clin Endocrinol Metab. 2001 Oct;86(10):5067-70.) 24 males to either receive ibuprofen, acetaminophen or a control after resistance excercise workout (10 - 14 sets). It showed that acetaminophen and ibuprofen had an impact on prostoglandins (in equal degree). Researchers concluded this impact on prostoglandins could have a big impact on muscle growth. One year later, these same researchers published (Am J Physiol Endocrinol Metab. 2002 Mar;282(3):E551-6.) measuring protein synthesis after resistance workout. The study was done on 24 males who either took acetaminophen, ibuprofen, or control. Results showed that the placebo group had 75% higher protein synthesis at the skeletal muscle than the ibuprofen or acetaminophen group. These painkillers didn’t effect overall body breakdown, but did effect protein synthesis at the muscle.
Aspirin also has a effect on decreasing protein synthesis. (source: J Biol Chem. 2007 Apr;282(14):10164-71. Epub 2007 Feb 6.)