Side effects causally linked to the administration of a worming product are extremely low. In most cases, an adverse reaction to a wormer is likely linked to the impact the wormer has on the parasitic infestation.
Roundworms reside in the small intestine. When a horse is wormed, roundworms are killed which, in large numbers, can become an obstruction within the small intestine, leading to impaction colic. This scenario most often affects young horses or those that haven’t been de-wormed regularly.
Veterinarian advice should be sought before administering worming products.