The Side Effects of Sentinel for Dogs

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Sentinel is a flea/heartworm preventative and wormer.

Sentinel is the brand name of a prescription flea and worm control product manufactured by Novartis Animal Health. The active ingredients of the product are lufenuron and milbemycin oxime. Laboratory testing results provided with the product insert of Sentinel claim a high rate of efficacy and safety, but side effects are still possible. If you suspect your dog is suffering a life-threatening or adverse reaction from Sentinel, speak to a licensed veterinary medical professional immediately.



Dogs who have contracted heartworm and do not test positive prior to beginning treatment with Sentinel may experience symptoms of hypersensitivity to the chemical milbemycin oxime. When the chemical meets the dead or dying microfilariae, or baby heartworms, the microfilariae release proteins into the bloodstream. This can cause a number of adverse symptoms, including vomiting, hypersalivation, trouble breathing and lethargy, according to the product insert for Sentinel for Dogs.


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Pruritus is the medical term for itchy skin. This itching is often uncontrollable and irritating or painful. Pruritis may result from the accumulation of dead or dying fleas, flea excrement or a sensitivity to Sentinel. Observe your dog and ensure that the itching does not become unbearable to the point of self-injury.



Urticaria, or hives, are indicators of an allergic reaction. They present as itchy, red welts and may be restricted to one area of the dog's body or the entirety of it. Hives are a rare side effect of Sentinel that did not manifest during clinical trials. If you observe hives on your dog, keep an eye out for other signs of an allergic reaction such as trouble breathing, swelling of the face, lips or tongue, biting at the paws, weakness and lethargy.



Diarrhea presented in just two of the 785 dogs tested during clinical trials of Sentinel. Because Sentinel is administered orally, it is possible for the drug to interact with stomach contents and cause stomach upset and loose stools. The drug may also alter the pH level of the stomach and cause an unpleasant buildup of gasses, leading to gastrointestinal problems. Be sure to give your dog plenty of fresh clean water and speak to a veterinarian if loose stools are present 24 to 48 hours after administering Sentinel.



Some dogs lose their appetite for several hours after taking Sentinel. This is not common, but if anorexia persists for more than five hours, seek medical advice.


Skin Congestion

Skin congestion is a broad term referring to the buildup of oils in and on the surface of the skin. Clogged pores result from skin congestion, causing pimples or blemishes to form. Again, skin congestion is a rare side effect, manifesting in only one of the dogs tested in clinical trials.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.


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