It is a well-known fact that adverse drug experiences can affect not only human beings, but also the animals, or even the environment. However, it is essential to evaluate the correlation between drug exposure and the adverse event.

Causality assessment is a method used for estimating the strength of the relationship between drug(s) exposure and the occurrence of adverse reaction(s). In the case of human beings, causality assessment is performed using any of the following scales or tools:

1. Naranjo’s Algorithm

2. WHO-UMC Assessment Scale

3. VCAT (versatile causality assessment tool)

In the case of Veterinary Medicinal Products (VMP), the preferred method of causality assessment is using the ‘ABON’ system, where

A – stands for Probable,

B – stands for Possible,

O – stands for Unclassifiable/insufficient data,

O1 – stands for Inconclusive,

N – stands for Unlikely

The guideline on harmonising the approach for causality assessment for adverse reactions to veterinary medicinal products came into effect in October 2004.

The six main factors that are considered for the causality assessment, are:

1. Associative Connection

2. Pharmacological and/or immunological explanation

3. Clinical or pathological phenomena,

4. Previous knowledge,

5. Other Causes, and

6. Reliability of data.

If the answers to the first four factors are Yes, then it is likely to be A or B, while a Yes for fifth and sixth factor, may mean otherwise.

With regards to causality assessment, no distinction is made if the product is used as per the reference safety information, or even if the use is considered off-label (unauthorized species, unauthorized route, or even unauthorized dose). Although causality assessment takes both product and treatment into consideration, the regulatory actions are triggered only by product-related causality.

Although one would agree that there is some amount of correlation between the causality for human and veterinary medicines, it may be prudent if a veterinarian is assigned the task of determining the causality.