What kills fleas on dogs instantly includes several vet-approved and effective treatments:

1. Prescription Oral Flea Tablets: Medications like Capstar (containing nitenpyram) start killing adult fleas within 30 minutes to four hours by interfering with the flea’s nervous system, causing rapid death. This is usually the fastest way to kill fleas on a dog.
2. Flea-Killing Shampoos: Medicated flea shampoos can kill adult fleas immediately upon use. These shampoos are applied by lathering the dog's coat, leaving it on for several minutes as directed, and rinsing thoroughly. They provide instant relief but do not offer long-term protection.
3. Fast-Acting Topical Spot-Ons: Products such as Advantage (imidacloprid) and Frontline (fipronil) kill fleas through contact within 12 to 24 hours. They are applied to the skin and spread naturally across the body to kill fleas.
4. Flea Combs: While not a chemical kill method, fine-toothed flea combs can instantly remove adult fleas physically from the dog's fur.

Among these, prescription oral tablets like Capstar are known for the fastest onset of action, usually killing fleas within 30 minutes. Flea shampoos kill fleas on contact during the bath. Topical spot-ons work within hours but not as instantly as oral tablets. For immediate killing of fleas on dogs, oral tablets and medicated shampoos are the best options to consider, ideally followed by a longer-term preventive treatment to avoid reinfestation. Always consult a veterinarian for the safest and most appropriate treatment for your dog’s specific situation. Natural remedies such as apple cider vinegar or lemon baths may help repel fleas but do not kill them instantly. This comprehensive approach ensures both instant flea killing and ongoing control for your dog’s comfort and health.