It is especially difficult to attribute an adverse reaction to a drug if the medical event is common in the user’s age group or in the condition being treated. Because the risk of heart attacks is high in older people, many of whom have arthritic pain requiring treatment with painkillers, it took six years to link Cox-2 inhibitors to a two- to three-fold increase in the risk of heart attacks.
如果该药品使用者分龄组中或该用药者的治疗状况中经常出现医疗事故，那么将副作用归咎于药品尤为困难。因为老年人突发心脏病的风险较高，他们中很多人都患有关节炎并需要止痛片的治疗，在六年光景之内，Cox-2止痛剂就会使心脏病突发可能性翻上两番或三番。