Lethargy, Weakness and More
Many drugs that pets are given have an effect on the brain. Sometimes this is the expected outcome (e.g., sedatives and anti-anxiety medications), but at other times it is an unwanted side effect. For example, opioids like morphine, butorphanol, buprenorphine, fentanyl and tramadol can cause lethargy or even dysphoria (abnormal sensations leading to a state of unease) in pets. Often these symptoms diminish over the course of a few days to a week or so as the body becomes used to the medication. The commonly prescribed antibiotic metronidazole is also known to have an effect on the brain, especially at high doses or in elderly patients. It can cause abnormal eye movements, weakness, unsteadiness and seizures.
Herding dogs such as Collies, Australian Shepherds, Shetland Sheepdogs, English Sheepdogs and related breeds, have a gene mutation that makes them especially sensitive to macrocyclic lactones, a class of parasiticides that includes ivermectin or moxidectin. Symptoms of toxicity include dilated pupils, unsteadiness, mental dullness, drooling, vomiting, blindness, tremors, seizures, coma and death. A genetic test is available to identify at risk dogs. It is important to note, however, that the dose of ivermectin or moxidectin in heartworm preventatives is so low that it is safe for use in any breed of dog.