Vomiting, lack of appetite, and loose stools are among the most common side effects noticed when a dog starts a new medication. It’s not unusual, and in many cases, the symptoms are temporary. But when reactions persist or worsen, it’s a sign that the medication may need to be adjusted—or in some cases, stopped entirely.

Dogs process medications differently based on age, breed, body weight, organ health, and existing conditions. Older dogs, for example, are more likely to struggle with liver or kidney function and may be more sensitive to commonly prescribed drugs.

Stomach Upset

Antibiotics, anti-inflammatories, thyroid medications, and even basic flea and worming tablets can cause gastrointestinal issues. Symptoms include nausea, vomiting, diarrhoea, bloating, or gas. Dogs with sensitive stomachs or conditions like pancreatitis are more likely to react poorly.

Some medications are better tolerated when given with food, while others should be administered on an empty stomach. If your dog refuses to eat or shows signs of discomfort after each dose, it’s worth consulting with your vet to review dosage and administration.

Changes in Behaviour or Mood

Some dogs become unusually quiet, anxious, or restless while on medications. Others may become sleepy or struggle to settle at night. Drugs like gabapentin, prednisone, antihistamines, and pain relievers can all cause behavioural changes.

Some dogs may even pace, whine, or pant excessively, particularly if they’re feeling disoriented or overstimulated. While mild drowsiness can be normal, anything that seems out of character should be monitored closely.

Drooling and Bad Taste Reactions

Bitter tablets like tramadol or metronidazole often trigger a strong drooling response. It’s especially common in smaller dogs or picky eaters. If the medicine can’t be hidden in food or a treat, some dogs will react with gagging, pawing at their mouth, or even vomiting.

Avoid crushing bitter tablets unless your vet specifically recommends it. Doing so usually makes the taste worse and increases the chance of rejection.

Skin Reactions

Topical flea treatments, injections, and even oral medications can lead to skin issues. You might notice hives, redness, scabbing, or bald patches. These reactions are usually localised but can spread if your dog scratches or licks excessively.

Breeds with sensitive skin—such as Staffordshire Bull Terriers or Boxers—may be more prone to visible irritation. A mild reaction may pass in a day or two, but ongoing irritation could require switching to an alternative product.

Liver and Kidney Strain

Most medications are metabolised by the liver and excreted through the kidneys. If these organs aren’t functioning properly, the drug can build up in the bloodstream, leading to more serious problems.

Look for:

• Yellowing of the gums or eyes (jaundice)

• Increased thirst and urination

• Vomiting, especially with lethargy

• Loss of appetite over several days

Regular blood tests are recommended for dogs on long-term medications like NSAIDs (non-steroidal anti-inflammatories), thyroid drugs, or anticonvulsants.

Neurological Symptoms

Seizures, tremors, and uncoordinated movement can occur with certain medications, especially if dosed incorrectly. Metronidazole is known to cause neurological side effects in older dogs or at higher doses. Some spot-on parasite preventatives have also been associated with shaking or twitching in sensitive individuals.

Collies, Australian Shepherds, and other herding breeds may carry a genetic mutation (MDR1) that affects how they process medications like ivermectin or loperamide. If you own one of these breeds, it’s worth discussing genetic sensitivity with your vet before starting treatment.

Fatigue and Lethargy

Fatigue is often seen with pain medications, sedatives, and antihistamines. While some drowsiness is expected, extreme tiredness or weakness can signal an adverse reaction.

If your dog is struggling to walk, standing still for long periods, or simply isn’t acting like themselves, you may be dealing with something more serious.

Unusual Bleeding or Bruising

Some antibiotics, chemotherapy drugs, or thyroid medications can affect blood clotting or platelet levels. Symptoms include nosebleeds, bleeding gums, bruises under the skin, or prolonged bleeding from minor cuts. These side effects are rare but require urgent attention.

If your dog is due for surgery or dental work, it’s especially important to disclose all medications to your vet in advance.

When to Be Concerned

Any sudden or severe reaction—especially involving difficulty breathing, collapse, disorientation, or seizures—requires immediate veterinary care. More subtle symptoms like decreased appetite or low energy may not seem urgent, but they can become serious if left unchecked.

In many cases, adjusting the dose or switching to an alternative medication can resolve the problem. Never stop a prescription suddenly without checking with your vet, as some medications must be tapered gradually to avoid withdrawal effects.

What to Do if Your Dog Reacts to Medication

1. Take note of symptoms, including when they started, and how long they’ve lasted.

2. Contact your vet—even for mild symptoms—before adjusting dosage or stopping medication.

3. Keep track of what your dog eats, how much water they drink, and how active they are.

4. Store medications safely and ensure doses aren’t accidentally doubled by another family member.

Summary

Most dogs will respond well to the medications prescribed by their vet—but not all. Being aware of common side effects and knowing what’s normal (and what’s not) can help you act quickly if your dog isn’t tolerating their treatment.

And if your dog belongs to a sensitive or high-risk breed like a Collie, Australian Shepherd, or Border Collie, be extra cautious and ask your vet about safer alternatives where needed.

For more guidance on breed-specific care, see our dog breeds section.