Checked against the SaluPaws toxic-food database
Caffeine is a methylxanthine — the same stimulant family as chocolate's theobromine — and dogs are far more sensitive to it than we are. A lick of latte is low-risk; coffee grounds, tea bags, energy drinks and caffeine pills are the emergencies. Signs start within 1–2 hours.
Mild signs can appear around 20 mg caffeine per kg of body weight; severe signs at higher doses.
Within 1–2 hours, lasting many hours:
Call your vet or an animal poison line (UK: Animal PoisonLine · US: ASPCA Animal Poison Control) with your dog's weight, the source (grounds, pills, energy drink), roughly how much, and when. Caffeine pills and grounds justify an immediate call even before any symptoms.
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Decaf still contains a little caffeine, but the dose in a lick or spill is negligible. It's still not a drink to share — milk fat and sugar don't help either — but it's not the emergency caffeinated coffee is.
Cup-for-cup tea has less caffeine, and a lap of milky tea is low-risk for most dogs. Chewed tea bags are the real hazard — concentrated leaves, plus a string-and-bag blockage risk for small dogs.
If ingestion was recent, vets typically induce vomiting and give activated charcoal, then manage heart rate, temperature and tremors with monitoring and medication until the caffeine clears. Early treatment shortens all of it.
Very similar — theobromine and caffeine are both methylxanthines, and chocolate contains some caffeine too. The symptoms and treatment overlap; caffeine tends to act faster. Our chocolate guide covers the dose maths for that side.
Sources: VCA Animal Hospitals — caffeine toxicity · MSD Veterinary Manual — methylxanthines. This page is general guidance, not a substitute for professional veterinary advice.