Veterinary medicines regulator updates how it calculates adverse event incidence rates
Source: gov.uk (2025-12-01)

VMD changes to calculation methods ensures figures reflect the way medicines are used.

The Veterinary Medicines Directorate (VMD) explains changes to how it calculates adverse event incidence rates for animal medicines, following updates to Veterinary Medicines Regulations in 2024. The changes will help to ensure the figures we report give a clear picture of real-world animal medicine use, making them easier to understand and compare.

The changes mean that previously published figures may be updated using new calculation methods, which could make them look very different to earlier versions – even if the actual number of adverse events reported hasn’t changed.

For example, under the previous method an incidence rate might have been shown as 0.0001, while under the new method we might report an incidence rate percentage of 0.01%. Both figures represent the same rate, but the percentage appears to be 100 times larger.

What are adverse events?

No medicine is 100% risk-free. Adverse events are observations that are unfavourable and unintended that happen after giving a medicine to an animal.

These events may be seen, for example, in the animal being treated, in other animals, the person handling the medicine or in the environment.

They include both reactions and times when the medicine has not worked as expected and can range from mild issues to more serious problems.

New reporting requirements for pharmaceutical companies

Under the updated regulations, companies that hold marketing authorisations for animal medicines must provide adverse event incidence percentages when submitting safety information to the VMD using the new calculations. Details of the calculations to be used can be found in section 4.4 of Guideline IV Signal management, including benefit-risk reports - GOV.UK. (https://www.gov.uk/government/publications/pharmacovigilance-of-veterinary-medicines-in-great-britain/periodic-safety-update-reports) Companies must also supply sales data at least yearly, which the VMD will use to estimate how many animals have been treated with each product.

How the VMD will now calculate adverse event incidence rates

The VMD will now calculate adverse event incidence rates using the formula:

Number of animals experiencing adverse events divided by the estimated number of animals treated, multiplied by 100 to give a percentage.

To estimate how many animals have been treated, the VMD will use sales figures combined with “dose factors” that account for how products are typically used according to their official instructions. This method is used as there is no system in place to gather usage data and recognises that some medicines require multiple doses per animal. This new method ensures adverse event incidence rates reported by the VMD reflect the way these medicines are used in practice.

The VMD has also changed which dates it uses for calculations. Where possible, it will now use the date when an adverse event started, rather than when it was reported to the VMD. This change aims to better reflect that some adverse events are reported long after they occur, however this approach is not suitable in all situations . Where this approach is not suitable, the VMD will give details on how a figure was calculated when publishing.

Further information on these incidence calculations, including how they can be used and what to consider when interpreting them, can be found in our new guidance on how the VMD calculates adverse event incidence rates (https://www.gov.uk/guidance/calculation-of-adverse-event-incidence-for-veterinary-medicines).

Safety of veterinary medicines

All veterinary medicines carry some risk of adverse events occurring, which the VMD categorises using an internationally recognised approach from “very common” (affecting more than 1 in 10 animals) to “very rare” (affecting fewer than 1 in 10,000 animals). The new calculation method will produce an incidence rate which will be used by the VMD to select the most appropriate frequency category. This could be for all adverse events or for a specific adverse event, depending on the nature of the publication, providing more accurate context for the likelihood of an adverse event occurring.

The VMD continues to encourage veterinary professionals and animal owners to report all adverse events, noting that regulatory decisions must be evidence-based and that underreporting can impact safety assessments.

Notes:
- Details on how to report can be found on Report a suspected problem with an animal medicine or microchip - GOV.UK. (https://www.gov.uk/report-veterinary-medicine-problem)
- All veterinary medicinal products (VMPs) authorised for marketing in the United Kingdom have their Summary of Product Characteristics (SPC) available on our Product Information Database, which can be accessed at https://www.vmd.defra.gov.uk/productinformationdatabase/.
- The SPC for a VMP includes information on what adverse events have been known to occur following administration of a particular product; depending on the SPC format these can be found in either section 3.6 Adverse events or 4.6 Adverse reactions.
- In addition to the information available on SPCs, the VMD may publish adverse event incidences for a specific active substance, a specific VMP, or groups of VMPs. Incidences may be published within safety updates added to the Urgent and clinically significant safety updates for veterinary medicines - GOV.UK. (https://www.gov.uk/guidance/urgent-safety-updates-for-veterinary-medicines) or within product/active substance-specific publications.