Serotonin syndrome in dogs is a serious drug reaction caused by too much serotonin activity in the nervous system. It usually follows a dog swallowing a serotonin-boosting medication or supplement, most often one prescribed to a person in the household, and it needs a veterinarian straight away.
It can be life-threatening, although death is uncommon. Owners of dogs on behavior medication should know the signs, and so should anyone whose own pills or supplements sit within a dog's reach.
What Is Serotonin Syndrome in Dogs?
Serotonin is a chemical messenger (neurotransmitter) the body makes on its own. Drugs and supplements described as serotonergic raise serotonin activity: they add raw material, release stored serotonin, or slow how fast the body clears it. When that activity overshoots, mental state, muscle control, and automatic functions such as heart rate, temperature, and gut movement all go off balance. That combination is serotonin syndrome.
In people, it usually comes from taking two serotonergic drugs together. In dogs, the most commonly reported route is accidental: a dog chews into a household member's antidepressant, ADHD medication, or supplement. It can also follow a dog's own medication being double-given, started, changed, or combined with another serotonergic drug.
Signs of Serotonin Syndrome in Dogs
Signs vary with the drug and the amount swallowed. Stomach upset is common, along with changes in behavior, muscles, and vital signs:
- Vomiting, diarrhea, or loss of appetite
- Drooling
- Restlessness, pacing, or panting
- Agitation
- Disorientation or confusion
- Dilated pupils
- Tremors or muscle jerks
- Muscle rigidity or difficulty walking
- Rapid heart rate (tachycardia) and rapid breathing (tachypnea)
- Fever (high body temperature)
- Depression or lethargy, the most common sign in a large study of dogs exposed to SSRI antidepressants
Severe cases can progress to seizures or coma. Both are emergencies.
Signs typically start within one to two hours of swallowing the drug. Slow-release (extended-release) tablets, made to dissolve gradually, can take 6 to 12 hours. When a dog gets a serotonergic medication every day, the effect can build over several days instead.
What to Do If You Think Your Dog Has It
If your dog has signs, or you find a chewed blister pack or open pill bottle, act now rather than waiting to see what happens:
- Call your veterinarian or the nearest emergency clinic. If you can't reach them, call a 24/7 animal poison line:
- ASPCA Animal Poison Control Center: (888) 426-4435 (a consultation fee may apply)
- Pet Poison Helpline: (855) 764-7661
- Bring the packaging. Take the bottle, box, or blister pack, plus a list of every medication and supplement your dog takes or could have reached, including over-the-counter and herbal products.
- Don't make your dog vomit unless your veterinarian or a poison line tells you to. Vets avoid it in dogs already showing signs, because the dog could inhale the vomit.
- Don't stop or change your dog's prescribed medication on your own. Suddenly stopping a serotonergic drug can cause problems of its own. Your veterinarian decides whether to pause, change, or continue it.
What Causes Serotonin Syndrome in Dogs?
Most cases trace back to one of four situations:
- The dog gets into someone's pills. A person's antidepressant, ADHD medication, pain medication, or supplement left on a counter or in a bag is the classic scenario, and a dog already on its own serotonergic medication can react more strongly.
- Two serotonergic products are combined. A dog on one behavior drug is prescribed another, or an owner adds a supplement at home. Every veterinarian who treats your dog needs its full medication and supplement list to catch this.
- A dosing mistake. Someone gives the dog's tablet twice, or the dog gets into its own medication. Your veterinarian chooses the product and dose; if a dose seems off, call them before giving it.
- A medication is started or changed. Tell your veterinarian promptly about new restlessness, vomiting, or tremors after a new serotonergic drug or a change to an existing one.
Behavior medications are a common part of treating problems like separation anxiety, so many homes keep serotonergic medicines around for both the dog and the people.
Drugs and Supplements That Can Cause It
Several unrelated drug families raise serotonin activity. The table groups the ones most relevant to dogs; the examples are not a complete list.
Group | Examples | How a dog usually gets it | Key point |
|---|---|---|---|
SSRIs (selective serotonin reuptake inhibitors) | Fluoxetine, sertraline, citalopram | Own prescription or a person's antidepressant | Usually milder in overdose than older antidepressants |
Tricyclic antidepressants (TCAs) | Amitriptyline, clomipramine | Own prescription or a person's medication | Also act on other brain chemicals, so overdose can cause more than serotonin signs |
Other antidepressants | Trazodone, mirtazapine | Own prescription, often for anxiety or sedation; a person's medication | Neither is a TCA; trazodone's risk rises when combined with other serotonergic drugs |
MAOIs (monoamine oxidase inhibitors) | Selegiline | Own prescription for canine cognitive dysfunction | Risk mostly comes from combining with other serotonergic drugs |
Opioid pain medication | Tramadol | Own prescription or a person's medication | Often prescribed alongside trazodone after surgery, a combination vets watch for |
Amphetamines and ADHD drugs | Amphetamine-based ADHD medications | Almost always a person's medication | Not commonly prescribed to dogs; call about any exposure |
Supplements | 5-HTP (from Griffonia seed extract); St John's wort | Bottles in the home or added by an owner | 5-HTP can cause severe poisoning on its own |
Some details matter more than the table can show.
SSRIs block the reabsorption (reuptake) of serotonin, so more of it stays active between nerve cells. In a review of 313 dogs reported to a pet poison helpline after SSRI exposure, about three in four (76.3%) showed no signs at all. Among those that did, nervous-system signs dominated, with depression or lethargy the most common. All 136 dogs with complete follow-up survived.
Trazodone is often grouped with TCAs, even by veterinary sources, but it is a serotonin antagonist and reuptake inhibitor (SARI). Mirtazapine is a separate type of antidepressant too. Neither behaves like amitriptyline or clomipramine in overdose or in its interactions.
Amitraz, the active ingredient in some tick collars and dips, also inhibits monoamine oxidase. A dog on trazodone or another serotonergic drug that chews an amitraz collar faces an interaction risk.
5-HTP (5-hydroxytryptophan) is the direct building block of serotonin and is sold as a sleep and mood supplement. In a study of 21 dogs that ate 5-HTP products, 19 developed signs, most often vomiting or diarrhea, seizures, and fever, and 3 died. Of the 17 dogs with signs that were treated, 16 recovered.
St John's wort can add to serotonin activity, but Merck notes problems typically arise only when large amounts are swallowed together with other drugs.
How Vets Diagnose It
Vets diagnose serotonin syndrome from two things together: a known or likely exposure to a serotonergic product, and signs that fit the picture. That's why the packaging and your full medication list are so useful.
When nobody saw what the dog swallowed, the vet may send blood, urine, or stomach contents to a toxicology lab. Results can take days, so treatment starts based on the signs while the lab works.
The signs overlap with other poisonings and illnesses and differ from drug to drug, so tell your vet about every medication and supplement in the house, not just your dog's own.
How Serotonin Syndrome Is Treated
There is no home treatment for serotonin syndrome. Everything below is done by a veterinary team.
Decontamination. If the drug was swallowed recently and the dog is not yet showing signs, the vet may induce vomiting and then give activated charcoal, which binds drug left in the gut. Once a dog has signs such as tremors, agitation, or seizures, vets generally skip this step because of the risk of inhaling vomit.
Supportive care. Treatment follows the signs:
- Anti-nausea and gut-protecting medicines for vomiting and diarrhea
- Muscle relaxants for tremors and rigidity
- Anti-seizure medication
- Intravenous (IV) fluids
- Cooling for dogs with a high temperature
- Monitoring of heart rate and rhythm, blood pressure, and temperature
Vets also often give cyproheptadine, a serotonin antagonist that blocks serotonin receptors, to counter the excess. Your veterinarian decides whether to use it and how much.
Severely affected dogs can need several days in the hospital.
Recovery and Prognosis
The outlook is good for most dogs that reach a vet quickly. Mildly affected dogs treated promptly often recover fully within 36 to 48 hours. The SSRI and 5-HTP studies above point the same way: most dogs recover, and deaths are uncommon.
The prognosis becomes guarded when signs are severe, when the dog swallowed a large amount or a more dangerous drug, or when treatment is delayed.
Serotonin Pathophysiology Explained
For the vet techs and students: serotonin (5-hydroxytryptamine, 5-HT) is made from tryptophan, with 5-HTP as the intermediate step. Neurons store it in vesicles, release it into the synapse, and recycle it through the serotonin transporter (SERT).
Serotonin also helps regulate gut function, blood vessel tone, clotting, body temperature, mood, sleep, and pain perception. That spread explains the mixed picture of the syndrome. Each drug group pushes on a different step (precursors, release, reuptake, breakdown), so combining groups stacks their effects.
Preventing Serotonin Syndrome
- Keep every medication and supplement, yours and your dog's, closed and out of reach, including in bags, on nightstands, and in pill organizers.
- Give your dog's medication exactly as prescribed, and check the label each time.
- Give every veterinarian who treats your dog a full list of its medications and supplements, including herbal products.
- Ask your vet before adding any supplement for a dog on a behavior medication.
- Never stop or change a prescribed medication without talking to your veterinarian.
Serotonergic medications help many dogs with anxiety and behavior problems, and fear of serotonin syndrome is not a reason to refuse one your veterinarian recommends. Most cases come from accidental access and unplanned combinations, which owners can largely prevent.
Originally written by Helen Roberts. Updated by the I Love Veterinary editorial team.
Sources
- Merck Veterinary Manual: Toxicoses in Animals From Human Antidepressants, Anxiolytics, and Sleep Aids
- Merck Veterinary Manual: Toxicoses in Animals From Human Dietary and Herbal Supplements
- JAVMA (Gwaltney-Brant et al., 2000): 5-Hydroxytryptophan toxicosis in dogs, 21 cases
- JVECC (Thomas et al., 2012): Retrospective evaluation of toxicosis from SSRI antidepressants, 313 dogs
- Veterinary Partner (VIN): Trazodone HCL
- Merck Veterinary Manual: Triazapentadiene (Amitraz) Toxicosis in Animals
- dvm360: Recognizing and treating serotonin syndrome
- VCA Animal Hospitals: Serotonin Syndrome
- ASPCA: Animal Poison Control
- Pet Poison Helpline: Are Antidepressants Toxic to Pets?
This article is for general information and is not a substitute for advice from your veterinarian.
