How a small dog's size magnifies the risk
At 8 kg, your terrier falls into the high-risk category for melatonin overdose—the 5 mg tablet represents over half the human dose, scaled down to a much smaller body. Unlike humans who use melatonin occasionally, dogs have no natural tolerance for this hormone, and their reactions can be extreme even without mould complicating matters.
The moisture that allowed mould to grow may have also partially dissolved the tablet, making absorption faster than with a fresh pill. Watch for exaggerated swallowing motions or head bobbing—early signs that the sedative effects are taking hold.
When drowsiness turns dangerous
While you might expect sleepiness from melatonin, in dogs it often manifests as disorientation first. Your terrier may stumble into walls or seem unable to judge distances. This ataxic state frequently progresses to full collapse, where the dog cannot right itself when laid on its side.
Paradoxically, some dogs experience the opposite reaction—restlessness, whining, and even elevated heart rate. The mould introduces variables that make the reaction unpredictable, as mycotoxins can either amplify or contradict melatonin's effects.
Why overnight exposure matters
The hours the tablet spent exposed to air and humidity allowed both mould growth and chemical changes in the melatonin itself. Degraded supplements can form new compounds with unknown effects, while the visible mould indicates bacterial activity that may have started breaking down the pill's fillers and binders.
Your dog isn't just ingesting melatonin at this point—it's consuming an uncontrolled mix of byproducts. Gastrointestinal upset becomes nearly guaranteed, adding vomiting or diarrhea to the list of concerns beyond neurological symptoms.
Emergency actions versus home monitoring
This isn't a "wait and see" scenario—the combination of factors (small dog size, full human-strength dose, mould contamination) creates too many variables for safe home observation. While inducing vomiting might seem logical, it's contraindicated with potential neurological symptoms, as aspiration risk increases when a dog's gag reflex is compromised.
Your veterinary team will likely administer intravenous lipids to bind the fat-soluble melatonin and provide cardiovascular support while monitoring for seizures. The window for most effective treatment begins closing within 30-60 minutes post-ingestion.
The step-by-step response, from the first minute to the vet visit, is in our Sleep Aids (Melatonin, Zolpidem, Valerian) emergency guide.