TABLE 1.

WHO PROBABILITY ASSESSMENT (CAUSALITY ASSESSMENT) SCALE FOR ADVERSE DRUG REACTIONS

Category	Description
1. Certain	A clinical event, including laboratory test abnormality, that occurs in a plausible time in relation 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.
2. Probable	A clinical event, including a laboratory test abnormality with a reasonable time relation to administration of drug, unlikely to be attributed to concurrent disease or other drug or chemical and which follows a clinically reasonable response on withdrawal (dechallenge). Rechallenge information is not required to fulfil this definition.
3. Possible	A clinical event, including laboratory test abnormality, with a reasonable time relation to administration of the drug, but which could also be explained by concurrent disease or other drug or chemicals.
4. Unlikely	A clinical event including a laboratory test abnormality, which makes a causal relation improbable, and in which other drugs, chemical or underlying diseases provide plausible explanations.
5. Conditional/unclassified	A clinical event, including a laboratory test abnormality, reported as an adverse reaction, about which more data are essential for a proper assessment, or the additional data are being examined.
6. Inaccessible/unclassifiable	A report suggesting an adverse reaction that cannot be judged because information is insufficient or contradictory and cannot be supplemented or verified.

As per WHO causality assessment criteria, ADR were classified as Certain, Probable, Possible, Unlikely, Conditional/unclassified and Inaccessible/unclassifiable.