WHO Causality Classification

WHO Causality Classification
Term Definition Source

Certain A clinical event, including laboratory test abnormality, occurring in a plausible time relationship to drug administration, and which cannot be explained by concurrent disease or other drugs or chemicals. The response to withdrawal of the drug (dechallenge) should be clinically plausible. The event must be definitive pharmacologically or phenomenologically, using a satisfactory rechallenge procedure if necessary. WHO letter ref.: M1O/372/2 (A) (1991)
Probable A clinical event, including laboratory test abnormality, with a reasonable time sequence to administration of the drug, unlikely to be attributed to concurrent disease or other drugs or chemicals, and which follows a clinically reasonable response on withdrawal (dechallenge). Rechallenge information is not required to fulfill this definition.
Possible A clinical event, including laboratory test abnormality, with a reasonable time sequence to administration of the drug but which could also be explained by concurrent disease or other drugs or chemicals. Information on drug withdrawal may be lacking or unclear.
Unlikely A clinical event, including laboratory test abnormality, with a temporal relationship to drug administration which makes a casual relationship improbable, and in which other drugs, chemicals or underlying disease provide plausible explanations. Would be not reportable
Not Related Any reaction that does not meet the criteria above. Add on . Not reportable