"Adverse reactions" refers to negative or unwanted effects that can occur when someone takes a medication, receives a medical treatment, or is exposed to a certain substance. Full definition
A number of adverse reactions in cats were the result of the cat either being treated with a product intended for dogs, or through exposure to a treated dog. (healthypets.mercola.com)
The treatment was well tolerated by patients and there were no serious adverse reactions reported from the drug. (sciencedaily.com)
For example, a treatment that works super well for one cat may cause adverse reactions for another. (pesthacks.com)