SSRI and SNRI antidepressants in Pets

SSRI and SNRI antidepressants can cause fatal serotonin syndrome, tremors and fever in pets. Check toxic doses, feline risks, and urgent care protocols.

What It Is

Selective serotonin reuptake inhibitors (SSRIs, including fluoxetine, sertraline, citalopram, and paroxetine) and serotonin-norepinephrine reuptake inhibitors (SNRIs, such as venlafaxine and duloxetine) are widely prescribed human medications for depression, anxiety, and neuropathic pain. They function by blocking the presynaptic reabsorption of serotonin (and noradrenaline), increasing neurotransmitter concentrations in the central and autonomic nervous systems. In domestic pets, an overdose causes massive serotonergic overstimulation, triggering a life-threatening neuromuscular and autonomic crisis known as serotonin syndrome.

Where Pets Come Across It

Most exposures occur indoors when dropped pills are snatched from bedroom floors, or when dogs chew through bedside pill organisers and prescription bottles. Cats are especially prone to ingesting venlafaxine (Effexor XR) capsules, because they appear attracted to the smell or texture of the gelatin capsule shell.

Which Animals Are Affected

Dogs and cats are both affected. Dogs usually chew into bottles. The ASPCA notes that cats appear particularly attracted to venlafaxine capsules and may eat them readily if they find them.

Published Toxic Dose

There is no single toxic dose that applies to all antidepressants. SSRIs such as fluoxetine, sertraline and citalopram, and SNRIs such as venlafaxine and duloxetine, differ in strength, and several are prescribed to dogs and cats by vets at carefully chosen doses. Merck notes that an overdose of most antidepressants can cause vomiting, lethargy, wobbliness, agitation and, in more serious cases, seizures or serotonin syndrome. Because the margin depends on the drug and the pet, call a vet or a poison line with the drug name, strength and number of tablets.

Signs and How Soon They Appear

Signs often start within a few hours, later with extended-release products. A pet may be either unusually sleepy or agitated, with dilated pupils, vocalising and vomiting. Serotonin syndrome adds tremors, muscle rigidity, a fast heart rate, high body temperature and sometimes seizures.

What to Do Right Now

Contact your veterinary clinic or an emergency animal poison helpline immediately. Do not attempt home emesis, particularly if the pet is showing tremors, disorientation, or agitation. Bring the medication bottle to confirm the exact active molecule and milligram strength. Veterinary intervention includes intravenous fluid therapy, active cooling, muscle relaxants (such as methocarbamol for tremors), anticonvulsants, and specific serotonin antagonists such as cyproheptadine.

Why cats eat venlafaxine capsules

Most human medicines are unappealing to cats, but the ASPCA reports that cats appear particularly attracted to venlafaxine (Effexor) capsules. The reason is not fully understood. Keep antidepressants in a closed cupboard, pick up any dropped capsule at once, and remember that pill organisers left open on a table are easy for a cat to raid.

Frequently Asked Questions

Sources and References

• Merck Veterinary Manual: Toxicoses From Human Antidepressants, Anxiolytics and Sleep Aids
• ASPCA: Antidepressant Toxicity

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