Reporting of suspected cases of lack of efficacy to wormers and other anti-parasitic veterinary medicines

Notice type: Advisory

Date: 24/03/2023

The HPRA operates an adverse reaction monitoring service in relation to veterinary medicines, This system is open not only to reports about adverse effects to the products, but also to reports of possible cases of lack of efficacy (i.e. the medicine does not appear to work).

Product name or type:
Wormers, anti-parasitic veterinary medicines

Target Audience:

Veterinary practitioners, pharmacists, licensed merchants, farmers, horse owners, pet owners etc.

Problem Or Issue:
Resistance in food-producing animals to wormers and flukicides has been reported from farms both in Ireland and globally, while lack of efficacy is sometimes observed following treatment with anti-parasitics in farm animals, including horses. Lack of efficacy may be due to many factors e.g., incorrect dose, incorrect choice of drug, re-infestation of animals, poor treatment application etc.  

Sometimes, it is due to the development of resistance in the target parasites. Resistance builds quite slowly on farms over several years and it might not always be recognised or reported. With increasing use of faecal-egg counts as a means for checking for worm resistance on Irish farms, any reports of resistance should be communicated to the HPRA using the ADR reporting system.

Unless reports of lack of efficacy are made, the benefit risk of the product is deemed not to have changed and necessary improvements to the terms of the marketing authorisation are not initiated. Moreover, in the absence of reports of lack of efficacy, vets might not be permitted by DAFM to import medicines under special licences, which might have better efficacy rates.

Background Information Or Related Documents:

Information on antiparasitic resistance is available from a number of sources e.g. - Antiparasitic Resistance (, report_final_2019-12-6.pdf (,

What is anthelmintic resistance? | SCOPS, Parasite Control TASAH Consult (Cattle & Sheep) - Animal Health Ireland.

CANTER – controlling antiparasitic resistance in equines responsibly.

Actions To Be Taken:

Vets should report lack of efficacy to either the HPRA or to the Marketing Authorisation Holder (MAH) for the medicine involved. The MAH is obliged to upload the report to the common database for reports in the EU, from which the HPRA can review the information.

Farmers should ask their vet to report any issues of product failure to the HPRA or the MAH.

Any adverse events occurring following use of veterinary medicinal products should be reported to the HPRA via the online reporting form available at or to the marketing authorisation holder of the relevant product(s), at the earliest opportunity.

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