Pets can experience side effects to medications they are given, just like people. These can range from mild to more severe reactions, depending on many factors. It’s always best to discuss the risks of a drug with your veterinarian and decide if the potential benefits outweigh the potential side effects. If you suspect your pet is having any side effects, speak with your veterinarian as soon as possible to determine the next steps. Common Medication Side Effects in Pets Symptoms of a drug reaction in pets, or side effects to that drug, can be based on: The type of medication and how it’s administered. Usually, injectable medications have a higher chance of causing side effects than oral medications. Your pet’s health status and age. Older pets that have a medical diagnosis might react differently to the same medication given to a young, healthy pet. These are some of the most common side effects that pets experience: Gastrointestinal Upset One of the most common side effects of pet medications is mild stomach upset. This is because most medications are taken by mouth and absorbed in the gastrointestinal tract. Some pets have a more serious reaction, including vomiting, diarrhea, and decrease or lack of appetite. These symptoms often resolve as their body adjusts to taking the medication, but sometimes the symptoms can become severe. Ask your veterinarian if the medicine should be given with or without food, because that can make a big difference on how your pet tolerates the medication. Some medications that commonly cause stomach upset are: Antibiotics. Consider giving your pet probiotics when they are on antibiotics, to prevent or relieve antibiotic-associated diarrhea, vomiting, and appetite issues. Levothyroxine (used for hypothyroidism in dogs) Methimazole (used for hyperthyroidism in cats) Steroids such as prednisone and prednisolone Blood pressure medications Gabapentin (especially in cats) Oral and topical flea and tick medications Excessive Salivation Excessive saliva production, or hypersalivation, is a common side effect with some pet medications. Often this is because the medicine has a bitter or unpleasant taste. For these medications, it is not recommended that the pet parent crush the medication, which can make it taste much worse. Tramadol, a pain medication, is a very commonly prescribed pet medication that’s known to have a bitter taste; it should not be crushed. Medications such as antibiotics, antihistamines, and steroids can cause dry mouth, which triggers saliva production. Other medications known to cause hypersalivation include methimazole, flea and tick medications, and gabapentin. Stomach Ulcers Nonsteroidal anti-inflammatory drugs (NSAIDs) such as carprofen (Rimadyl), deracoxib (Deramaxx), meloxicam, and robenacoxib (Onsior), as well as steroids like prednisone and prednisolone, are known to increase your pet’s risk of getting gastrointestinal ulcers. These ulcers can then cause vomiting and diarrhea, with or without blood and/or dark, tarry stools. These medications should NEVER be given together, as it greatly increases the risk of ulcers. Stomach ulcers are often treated with medications that reduce acid production and protect the lining of the gastrointestinal system. Lumps and Bumps Side effects of medications that affect the skin are very common among pets. Lumps and bumps on the skin are most often caused by injectable medications. After the injection, you may notice a small lump where the medication was administered. These lumps often go away quickly as the medication is absorbed into your pet’s body. Skin Irritation Oral, injectable, and topical pet medications can all cause skin irritation. Antibiotics, thyroid medications, steroids, and flea and tick medications have all been associated with skin irritation in pets. Hair loss, itchiness, swelling, redness, scabbing, red bumps, hives, and blisters are the most common symptoms of skin irritation in pets. Skin reactions vary highly in severity and level of pain. They can be mild and resolve easily, or they can be severe, leading to immune system conditions such as lupus. Skin irritation in pets can also cause intense licking, biting, and chewing, which can cause worse damage to your pet’s skin. Liver or Kidney Damage Any medication your pet receives needs to be broken down by the body to be effective. The liver and kidneys do this, and they can be damaged in the process. Common side effects are increased thirst and urination, vomiting, diarrhea, yellowing of the skin or eyes, and lethargy. NSAIDs used short- or long-term are commonly associated with liver and/or kidney damage. This reaction is commonly called idiosyncratic, which means it is unpredictable. In general, if an NSAID medication is given at the correct dose for the correct period of time, these side effects should not occur. Steroids are known to cause liver issues, but usually only when given at high doses for long periods of time or if the liver was already compromised before starting the medication. Blood pressure medications have also been known to cause kidney dysfunction. Lethargy or Behavioral Changes Pet medications often affect a pet’s brain, causing them to behave differently. Sometimes this behavior change is what they were prescribed for, such as with anti-anxiety and sedative/pain medication such as opiates, tramadol, and gabapentin. However, other times it’s an unintended side effect, like restlessness and hyperactivity. These behavior changes can be mild to severe, even causing dysphoria. Dysphoria is an altered mental state where your pet might vocalize, pant, or have difficulty settling down. Dysphoria is also common after or during anesthesia in pets. Metronidazole, which is used to treat infections, is an antibiotic known to have effects on the brain in older pets or if too high a dose is given. Most antihistamines, which are commonly used for a wide variety for allergy manifestations, can also cause lethargy or hyperactivity, depending on the pet. Other medications known to have these side effects are levothyroxine, steroids, blood pressure medications, insulin, antihistamines, and flea and tick medications.