Both Dolonex IM Injection and nimesulide are pain killers. However, Dolonex IM Injection is a non-selective COX (cyclooxygenase -an enzyme responsible for inflammation and pain) inhibitor whereas nimesulide is selective to the COX-2 enzyme. This selectivity of nimesulide is supposed to decrease the side effects as compared to piroxicam. It should not be used in children below 12 years of age.