💊 Dog Medications & Side Effects
When your vet prescribes medication, knowing what it does, what side effects to expect, and what combinations to avoid makes you a safer, more informed carer. A plain-English guide to the most common drugs prescribed for UK dogs.
🐾 Before You Start — Key Principles
Veterinary medications are remarkably safe when used correctly — but "correctly" requires more than just giving the right pill at the right time. It means understanding what to watch for, knowing which combinations are dangerous, and recognising when a side effect is minor and expected versus a signal to stop and call the vet immediately.
This guide covers the medications most commonly prescribed in UK veterinary practice. It is not a substitute for your vet's specific instructions for your individual dog — but it should help you ask better questions and make better observations.
Ibuprofen, paracetamol (acetaminophen), naproxen and aspirin are all toxic or potentially fatal to dogs at human doses. Ibuprofen and naproxen cause severe gastrointestinal bleeding and kidney failure. Paracetamol causes liver failure. Even a single standard human dose can kill a small dog. If your dog is in pain, call your vet — do not reach for the medicine cabinet.
This includes all prescription medications, over-the-counter products, supplements, joint support chews, flea and tick treatments, and herbal remedies. Many dangerous drug interactions in dogs occur because owners assume that supplements or flea treatments don't count as "medications." They do.
💊 The Most Common Drug Classes
NSAIDs — Non-Steroidal Anti-Inflammatory Drugs
What they're for: Pain and inflammation — most commonly osteoarthritis, post-surgical pain, soft tissue injuries and dental pain. NSAIDs are among the most frequently prescribed medications in UK veterinary practice and, when used correctly, are highly effective and generally well-tolerated.
How they work: They block enzymes (COX-1 and COX-2) involved in producing inflammatory chemicals called prostaglandins. Unfortunately, the same enzymes also protect the stomach lining and support kidney blood flow — which explains why the stomach, kidneys and liver are the main targets for side effects.
Common side effects
- Vomiting or nausea
- Diarrhoea
- Reduced appetite
- Lethargy in first few days
Stop immediately and call your vet if you see
- Black, tarry stools (melena)
- Vomiting blood
- Yellowing of gums or eyes (jaundice)
- Marked increase in thirst or urination
- Severe lethargy or collapse
Important rules: Always give with food unless your vet specifically advises otherwise. Never give two different NSAIDs at the same time. Never give with steroids — this combination significantly increases the risk of gastrointestinal ulcers. Dogs on long-term NSAIDs should have blood work (kidney and liver function) checked at least every six months. If switching between NSAIDs, a washout period of several days is required.
Corticosteroids (Steroids)
What they're for: A very wide range of conditions — allergic reactions, skin disease, inflammatory conditions, immune-mediated diseases (where the immune system attacks the body's own tissues), some cancers, Addison's disease, and short-term severe inflammation. Prednisolone is the most commonly prescribed steroid in UK veterinary practice.
How they work: Corticosteroids are synthetic versions of cortisol — the body's own stress hormone. They powerfully suppress inflammation and the immune system by blocking multiple inflammatory pathways simultaneously, which makes them more broadly effective than NSAIDs but also more likely to produce significant side effects, particularly with longer courses.
Very common side effects (expected)
- Increased thirst and urination (PUPD) — affects most dogs; a UK VetCompass study found this in 39% of side-effect cases
- Increased appetite and begging
- Panting, especially at night
- Restlessness or behavioural changes
- Weight gain on longer courses
Signs of more serious problems
- Vomiting blood or black stools (GI ulcer)
- Extreme muscle weakness or pot-bellied appearance (long-term use)
- Urinary tract infections (UTIs occur in up to 30% of long-term cases)
- Coat changes — thinning, hair loss, skin fragility
- Signs of Cushing's disease with prolonged high doses
Critical rule — never stop abruptly: Dogs on steroids for more than a few weeks must be tapered off gradually, not stopped suddenly. Abrupt withdrawal can cause an Addisonian crisis — a life-threatening collapse of the adrenal glands. Always follow your vet's tapering schedule exactly. Never combine with NSAIDs.
Antibiotics
What they're for: Bacterial infections — skin infections (pyoderma), urinary tract infections, respiratory infections, gastrointestinal infections, dental infections, wound infections and ear infections. Antibiotics have no effect on viral infections such as parvovirus or kennel cough caused by viruses.
Common side effects
- Vomiting, diarrhoea or soft stools — very common; giving with food helps
- Reduced appetite
- Yeast overgrowth (ear or skin) with long courses
- Metronidazole specifically: neurological signs (wobbliness, abnormal eye movements, seizures) at high doses or in older dogs — call vet immediately
Important rules
- Always complete the full course — stopping early contributes to antibiotic resistance and may allow infection to return
- Give with food unless instructed otherwise
- Consider a probiotic supplement during and after the course to support gut flora — discuss with your vet
- Doxycycline: give with a full meal and ensure access to water; can cause oesophageal irritation if stuck in the throat
Gabapentin
What it's for: Chronic pain (particularly neuropathic pain and osteoarthritis pain that does not respond adequately to NSAIDs alone), seizure control (usually alongside phenobarbital), and pre-operative sedation and anxiety. Gabapentin has become one of the most commonly prescribed drugs in UK veterinary practice for chronic pain management and is frequently used safely alongside NSAIDs — unlike steroids.
Common side effects
- Sedation — often pronounced in the first few days, usually improves
- Wobbliness / loss of coordination (ataxia)
- Increased appetite
- Excessive salivation in some dogs
Important rules
- Do not use liquid human gabapentin formulations — many contain xylitol, which is toxic to dogs. Use only veterinary-prescribed products
- Taper dose when stopping long-term use rather than stopping abruptly
- Sedation effect can be useful for vet visits — some vets prescribe a single pre-visit dose for anxious dogs
Apoquel (Oclacitinib)
What it's for: Itching (pruritus) caused by allergic skin disease (atopic dermatitis). Apoquel works rapidly — most dogs see significant itch relief within 24 hours — and is widely used as an alternative to steroids for long-term allergy management. It works by blocking specific enzymes (JAK1 and JAK3) involved in the itch and inflammatory pathway.
Common side effects
- Vomiting, diarrhoea, reduced appetite — less common than with steroids or cyclosporine
- Lethargy
- Increased susceptibility to infections (skin, ear, urinary) due to partial immune suppression
- Rarely: bone marrow suppression (detectable on blood work only)
Important rules
- Not licensed for dogs under 12 months of age
- Long-term monitoring blood work is advisable
- Cytopoint (monthly injection) works differently and has a very low side-effect profile — an alternative worth discussing with your vet
- Does not combine well with immunosuppressive doses of steroids long-term
Seizure Medications
What they're for: Epilepsy and seizure disorders. Most epileptic dogs in the UK are managed on phenobarbital, often in combination with potassium bromide. These are lifelong medications — stopping them abruptly can trigger severe seizure clusters or status epilepticus (continuous seizuring), which is a veterinary emergency.
Common side effects
- Phenobarbital: sedation (especially initially), increased thirst, urination and appetite, wobbliness
- Phenobarbital long-term: liver enzyme elevation — requires regular blood monitoring
- Potassium bromide: sedation, increased appetite and thirst, wobbliness
- Levetiracetam: generally well-tolerated; mild sedation or behavioural changes
Critical rules
- Never miss a dose or stop abruptly — seizure risk is serious
- Blood level monitoring (therapeutic drug monitoring) is essential — phenobarbital requires regular serum level checks
- Salt intake affects potassium bromide levels — do not change diet significantly without discussing with your vet
- Keep a seizure diary — frequency, duration, recovery time
Cardiac Medications
What they're for: Heart disease — most commonly congestive heart failure (CHF), mitral valve disease (very common in Cavalier King Charles Spaniels and other small breeds), and dilated cardiomyopathy. These medications are typically lifelong and must be given consistently — irregular dosing can destabilise a dog whose heart disease is well-controlled.
Common side effects
- Furosemide (diuretic): markedly increased urination — ensure constant water access; electrolyte imbalances possible
- Vetmedin: loss of appetite, lethargy, diarrhoea in some dogs initially
- ACE inhibitors (benazepril, enalapril): reduced appetite, lethargy, kidney function changes
- All cardiac drugs: weakness, cold extremities, or fainting — contact vet immediately
Critical rules
- Never miss doses — cardiac stability depends on consistent blood levels
- Vetmedin should be given on an empty stomach (one hour before food) unless your vet advises otherwise
- Regular recheck appointments and blood/urine monitoring are essential
- Report any sudden changes in breathing, exercise tolerance or coughing promptly
⚠️ Dangerous Drug Combinations
The following combinations are associated with serious — sometimes fatal — outcomes in dogs. These are not theoretical risks. They are well-documented causes of veterinary emergencies that are often entirely preventable.
NSAIDs + Steroids
The most important combination to avoid in UK veterinary practice. Giving any NSAID (Metacam, Rimadyl, Previcox etc.) together with any corticosteroid (prednisolone, dexamethasone etc.) dramatically increases the risk of severe gastrointestinal ulceration — which can cause life-threatening bleeding or perforation. A washout period of at least five days is required when switching between the two classes.
Two Different NSAIDs Simultaneously
Combining two NSAIDs (for example, Metacam and Rimadyl, or Metacam and aspirin) multiplies the gastrointestinal and renal risk without adding any additional pain relief. Never give two NSAIDs at the same time, even if one was left over from a previous prescription.
Serotonin-Raising Drugs Together (Serotonin Syndrome)
Several drugs used in dogs increase serotonin levels in the brain — including some anti-anxiety medications (clomipramine, fluoxetine), the flea treatment amitraz (in some tick collars), and tramadol. Combining these can cause serotonin syndrome — a potentially fatal condition involving tremors, hyperthermia, seizures and cardiovascular collapse. Always tell your vet about all medications before adding anything new.
Human Liquid Gabapentin (Xylitol Risk)
Some human liquid gabapentin formulations contain xylitol as a sweetener — a compound that is highly toxic to dogs, causing rapid hypoglycaemia (dangerously low blood sugar) and liver failure. Never use human liquid gabapentin for your dog. Only use veterinary-prescribed gabapentin formulations.
NSAIDs in Dehydrated or Renally Compromised Dogs
NSAIDs reduce blood flow to the kidneys as part of their mechanism of action. In a well-hydrated healthy dog this is manageable, but in a dehydrated, anaesthetised or renally compromised dog it can cause acute kidney injury. This is why vets withdraw NSAIDs before surgery and why any dog on NSAIDs who stops drinking or becomes unwell should be seen promptly.
🚨 Side Effects That Always Need Immediate Veterinary Attention
Most medication side effects are mild and self-limiting — mild nausea, soft stools or initial sedation are common and manageable. The following are different. Any of these requires you to stop the medication and contact your vet or an emergency clinic the same day.
Black or Bloody Stools
Black, tarry faeces (melena) indicate digested blood from the upper GI tract — a sign of ulceration. Bright red blood indicates lower GI bleeding. Both are serious with any medication
Vomiting Blood
Fresh blood or "coffee ground" material in vomit indicates GI bleeding. Stop any NSAID or steroid immediately and call your vet
Yellowing of Gums or Eyes
Jaundice indicates liver involvement. Can occur with NSAIDs, some antibiotics, and other drugs. Requires urgent blood work
Seizures or Tremors
Neurological signs can occur with metronidazole at high doses, some flea treatments, and certain antiparasitic drugs. Veterinary emergency
Difficulty Breathing
Can indicate anaphylaxis (severe allergic reaction), which can occur with any medication. Most likely within the first hour of administration. Veterinary emergency
Sudden Collapse or Extreme Weakness
Can indicate cardiovascular effects, severe allergic reaction, or toxicity. Any dog that collapses on medication needs emergency assessment
Marked Increase in Thirst
Expected with steroids, but a dramatic new-onset increase in thirst on NSAIDs can indicate kidney stress. Requires vet assessment and blood work
Wobbliness or Loss of Coordination
Mild ataxia is expected with gabapentin initially. Sudden severe wobbliness or an inability to walk straight on any other medication warrants a call to your vet
📋 Practical Tips for Giving Medications
Getting your dog to take tablets
The most reliable method is hiding the tablet in a small amount of a high-value food — a pea-sized piece of soft cheese, peanut butter (check it contains no xylitol), cream cheese, or a piece of cooked chicken. Give a plain piece first, then the hidden tablet, then another plain piece to encourage swallowing without investigation. Pill pockets (commercially available soft treats with a hollow centre) work well for many dogs.
If your dog discovers and spits out tablets, ask your vet whether the medication can be compounded into a flavoured liquid or chewable formulation — many veterinary pharmacies in the UK offer this service.
Keeping track
For dogs on multiple medications or complex dosing schedules, a simple written chart on the fridge — medication name, dose, frequency, and time given — prevents accidental double doses or missed doses. This is particularly important for seizure medications where consistency is critical, and for dogs with heart disease where dose timing matters.
Storage
Most tablets should be stored at room temperature away from moisture and direct light. Liquid medications often need refrigeration — check the label. Keep all medications in their original packaging, which carries the dispensing label with your dog's name, dose, and expiry date. Never transfer medications between containers.
Missed doses
As a general rule: if you notice a missed dose within a few hours, give it immediately. If the next dose is due soon, skip the missed one and resume normally — do not double up. The exception is seizure medications, where consistency is particularly important — discuss a specific plan with your vet at the time of prescribing.
💊 The golden rule on monitoring: Any time your dog starts a new medication, watch closely for the first 48–72 hours and note any changes in behaviour, appetite, drinking, urination, or digestion. This is the window when most initial reactions occur. If something seems wrong, it is always better to call your vet and be reassured than to wait and see — particularly with NSAIDs and steroids, where early intervention prevents minor side effects becoming serious ones.