[Difficulties and bias in attributing side effects to drugs].

A number of difficulties and bias are known to make difficult the establishment of the relation cause/effect of side effects. Five sources of difficulty can be identified: the patient, the disease, the drug, the doctor, the studies and data available on side effects of drugs and which are used either as a model or to decide whether a side effect has occurred or not in a patient. All these difficulties and bias must be taken into account when attributing side effects of drugs.