Why diarrhea signals poisoning, not just allergy
Allergic reactions in cats typically cause facial swelling or hives rather than gastrointestinal distress. The diarrhea you're seeing suggests your cat's body is trying to expel the 5-HTP before more gets absorbed into their bloodstream. At 5 kg, your cat is large enough that a single lick might not cause seizures immediately, but the clock is ticking — neurological symptoms often follow digestive ones within 2-3 hours.
The itching could stem from two mechanisms: either filler ingredients like magnesium stearate irritating their system, or early neurological effects making them hypersensitive to touch. Unlike food allergies that take hours to develop, poisoning symptoms from 5-HTP usually appear rapidly — sometimes within 30 minutes of ingestion.
How neurological symptoms progress in cats
After gastrointestinal signs, cats often develop 'hyperesthesia' — exaggerated sensitivity to sound and touch. You might notice their skin rippling along the back or sudden aggressive reactions to gentle petting. These precede more dangerous symptoms like loss of coordination or muscle rigidity. The broken capsule means they ingested powder directly, which absorbs faster than an intact pill would.
Watch for subtle changes in pupil size or rapid blinking — these indicate the 5-HTP is affecting neurotransmitter balance. Unlike dogs, cats rarely pant when distressed, so increased respiratory rate might show as shallow, quick breaths through the nose. Any vocalization changes (excessive meowing or unusual silence) warrant immediate veterinary attention.
Why waiting risks rapid deterioration
5-HTP doesn't just cause discomfort — it can permanently alter brain chemistry at high doses. What begins as mild itching could progress to full-body tremors within an hour, especially in a cat this size. Their liver lacks certain enzymes to break down serotonin precursors efficiently, so the substance lingers longer than it would in humans.
Even if symptoms seem to stabilize, internal damage like elevated body temperature or irregular heartbeat might not be visible. Dehydration from diarrhea accelerates toxin concentration in the bloodstream. Don't be fooled if your cat seems to 'sleep it off' — this could indicate neurological depression rather than recovery.
Handling the itching without making things worse
Resist giving any human antihistamines — these can interact dangerously with serotonin overload. Wipe your cat's coat with a damp cloth to remove potential powder residue, especially around the face and paws where they groom. Use plain water, as soap or alcohol could increase absorption through the skin.
If they're scratching intensely, temporarily cover their claws with soft socks to prevent self-injury, but only if they'll tolerate this without stress. Increased agitation makes neurological symptoms worse. Your vet may administer muscle relaxants or IV fluids to flush the system, but these require professional calculation for feline metabolism.
The step-by-step response, from the first minute to the vet visit, is in our 5-HTP & Herbal Supplements emergency guide.