As a student athlete, I have noticed on a regular basis that a large number of my teammates and even just those I train with at the gym have the habit of taking painkillers prior to working out.
For those of you who do this too, try to avoid the NSAIDS (Non steroidal pain killers) and Acetaminophen when you are going to be active. If done consistently, it is very likely to cause a number of problems including:
GI Tract discomfort due to the blocking of necessary enzymes, Cyclooxygenase-1 and Cyclooxygenase-2
Increased risk of kidney damage due to the dehydration experienced during workouts. With less water available in the body, we have less available to run through in filtration efforts, thus creating a harsher response on the kidney as a result of the ibuprofen.
I would also recommend against taking Acetaminophen pre-workout for pain, as part of the side effects include a decreased ability to regulate core-temperature due to its effects on the Central Nervous System.
In summation of the above, if you have pain, try not to take pain medications prior to a workout, as there are a number of negative effects on the body associated with this. Taking an NSAID or Acetaminophen after workouts is all good, of course. Just be sure to rehydrate when doing so!
Source: Clinical Pathology for Athletic Trainers by Daniel P. O’Connor and A. Louise Fincher