Simple measures such as aspirin or Tylenol, with or without codeine, or ibuprofen may do the job well enough. But when pain is severe, the dosage has to be increased or the drug has to be taken more frequently. If these simple measures don't help, then it is important to increase the strength or potency of the medication. Sometimes, just the addition of an adjuvant medication is all that is needed. Not all people tolerate all drugs equally.
Some people are allergic to various medications.
Some develop side effects from medications that others taking the same drugs do not share. Some people tolerate one specific drug in a class of drugs but do not tolerate others in the same class. Some do not tolerate any drugs in a particular class. Everyone is an individual. While 90 to 95 percent of patients receive adequate pain control using the WHO guidelines, there are still 5 to 10 percent of patients who do not achieve adequate pain control.