A single 20 mg fluoxetine capsule is 5 mg/kg for a 4 kg cat, which is above the band where serotonin syndrome starts, and one 50 mg sertraline tablet is 12.5 mg/kg, which is intensive care territory. Signs usually begin at 30 to 60 minutes. The 6 hour mark is the decision point for most exposures, but not for fluoxetine, and the difference is the half-life rather than the dose.
One tablet is already a dose
Human antidepressant tablets are sized for a 70 kg adult, so the smallest pill on the shelf is 4 to 20 times a cat's weight in milligrams. Fluoxetine comes as 10 and 20 mg capsules, sertraline as 25, 50 and 100 mg tablets, citalopram and paroxetine as 10 and 20 mg, escitalopram as 5 and 10 mg. Venlafaxine, an SNRI that behaves much the same in overdose, comes in 37.5 to 150 mg.
Mirtazapine and fluoxetine at the amounts a veterinarian prescribes for a cat are not poisonings; the arithmetic has to include why the pill was in the house.
Dose thresholds in three bands
Veterinary toxicology treats SSRIs as a dose-dependent spectrum. Under 1 mg/kg: drooling, 1 or 2 vomits, mild lethargy, and nothing more in most cats. From 1 to 3 mg/kg: agitation, dilated pupils, tremor, pacing, a heart rate above 220. Above 3 mg/kg: the serotonin syndrome band, with hyperthermia over 39.5 degrees Celsius, rigidity and a seizure risk. Above 10 mg/kg, or any ingestion by a cat under 2 kg, treat as critical regardless of how well it looks at the door.
| Drug | Tablet (mg) | mg/kg at 4 kg | Tablets to 3 mg/kg | Half-life |
|---|---|---|---|---|
| Fluoxetine | 20 | 5.0 | 0.6 | 1-4 days |
| Sertraline | 50 | 12.5 | 0.24 | 1 day |
| Citalopram | 20 | 5.0 | 0.6 | 1-2 days |
| Paroxetine | 20 | 5.0 | 0.6 | 1 day |
| Escitalopram | 10 | 2.5 | 1.2 | 1-2 days |
| Venlafaxine | 75 | 18.8 | 0.16 | 12 hours |
The third column is the one to read at 2 a.m.: every row except escitalopram is already past 3 mg/kg from a single tablet in a 4 kg cat, and a 3 kg cat pushes sertraline to 16.7 mg/kg. Citalopram deserves a second look because it prolongs the QT interval more than the others, so a rhythm strip matters even when the behavior looks normal.
30 to 60 minutes: the gut speaks first
SSRIs are well absorbed and fast. Drooling and vomiting appear at 30 to 60 minutes, before any neurological sign, because the gut has more serotonin receptors than the brain does at that moment. Vomiting limits absorption and proves the tablet was real; it is not a good sign.
The neurological phase runs from 1 to 2 hours, and peak blood levels for most of these drugs land at 4 to 8 hours, later than owners expect. That is why a cat that is only drooling at hour 1 can be rigid and hot at hour 5.
Serotonin syndrome: three signs, not one
The triad is taught as neuromuscular, autonomic and mental, and a cat needs features from at least two of the three for the diagnosis to hold. Neuromuscular: a fine tremor that becomes a whole-body shake, hind limb rigidity, exaggerated reflexes, and in severe cases the head pressed back. Autonomic: pupils wide and fixed, heart rate above 240, temperature above 39.5 degrees Celsius, drooling, diarrhea. Mental: agitation that flips to staring, yowling at nothing, then depression.
The temperature separates a cat that needs a quiet room from one that needs a cold towel and a clinic. Above 40.5 degrees Celsius, muscle breakdown and clotting problems begin, and the cat should already be in a car.
The 6 hour line, and why fluoxetine ignores it
For sertraline, citalopram, paroxetine and venlafaxine, a cat with no sign at 6 hours after a dose under 1 mg/kg, confirmed by a vet, is unlikely to develop serotonin syndrome. That is the 6 hour line, and it is a discharge rule, not a home rule.
Fluoxetine breaks it. Its half-life in dogs runs 1 to 4 days, and its active metabolite norfluoxetine 4 to 16 days, with cat data thin, so a single capsule keeps producing effect for a week. A fluoxetine exposure needs observation out to 24 to 72 hours, and a cat that was fine at hour 6 can be tremoring on day 2.
What not to do
- Do not make the cat vomit at home. Serotonin syndrome lowers the seizure threshold, and a cat that seizes with its head down aspirates; vomiting is a vet decision made inside the first 60 to 120 minutes.
- Do not give tramadol, St John's wort, another SSRI, or a human sedative to settle it. Each adds serotonergic or sedative load, and the combination turns a tremor into a seizure.
- Do not exercise the cat or wrap it in blankets to stop the shaking. Muscle activity and insulation both push the temperature up, and hyperthermia is the part of this syndrome that kills.
What the clinic does, by name
Care is prescription-only. Inside the first 60 to 120 minutes a vet may give activated charcoal at 1 to 3 g per kg if the cat is alert and swallowing. After that the names are diazepam or midazolam for tremor and seizure control, cyproheptadine as a serotonin blocker, intravenous fluids, active cooling, and a beta blocker such as esmolol if the rate stays above 240. Every dose is calculated on the clinic scale and dispensed on a prescription.
Timeline from the tablet to 72 hours
| Time | What is happening | What you measure | Threshold |
|---|---|---|---|
| 0-15 min | Bag the blister pack, note the pill count | mg on the label, kg on the scale | mg/kg total |
| 30 min | Drooling, first vomit | Heart rate, gum colour | 220 bpm |
| 2 h | Tremor, dilated pupils | Rectal temperature | 39.5 C |
| 6 h | Decision line for most SSRIs | Gait, reflex, response to name | no sign at 1 mg/kg |
| 24 h | Fluoxetine still active | Temperature, appetite | under 39.5 C |
| 72 h | Norfluoxetine tail ends | Behavior back to baseline | eating 3 days running |
Do this now
- Count the pills, photograph the label, and note the clock time of the missing dose.
- Weigh the cat and divide the milligrams by the kilograms before you call.
- Take a rectal temperature and a 15 second heart count, then multiply by 4.
- Call the clinic, say SSRI, the drug name, the mg/kg and the minute; go in even if the cat looks normal.
- Keep the room at 20 to 22 degrees Celsius, no blankets, no exercise, and bring the packaging.
If the cat also chewed a houseplant in the same hour, treat it as two exposures and check the plant poisoning list. For any other substance on the shelf, the toxic dose tool gives the threshold by weight, and a cat that stops passing urine after days of poor intake needs the routine in catheter aftercare.
Frequently asked
My cat bit into one 20 mg fluoxetine capsule. Is that serious?
Yes, treat it as serious: 20 mg is 5 mg/kg for a 4 kg cat, above the 3 mg/kg band where tremor, hyperthermia and rigidity appear, and fluoxetine keeps acting for days because of its long half-life. Call the clinic, bring the capsule shell, take a rectal temperature, and expect observation out to 24 hours rather than a 6 hour discharge.
What exactly counts as serotonin syndrome in a cat?
Features from at least two of three groups. Neuromuscular means tremor, hind limb rigidity, exaggerated reflexes. Autonomic means fixed wide pupils, a heart rate above 240, a temperature above 39.5 degrees Celsius, drooling or diarrhea. Mental means agitation flipping into staring, yowling, then depression. A single dilated pupil in a cat that is otherwise normal is not the syndrome, but it is worth a phone call.
The 6 hour mark passed with no signs. Can I stop watching?
Only for sertraline, citalopram, paroxetine and venlafaxine at a dose under 1 mg/kg, and only with a vet's agreement. Fluoxetine and its active metabolite keep working for days, so that exposure needs a 24 to 72 hour watch with a temperature check twice a day, and any return of tremor, rigidity or a temperature above 39.5 degrees Celsius means going back in.