How Oral Flea Medications Work

by JustGreatTips

Your dog can swallow a pill that will kill adult fleas. It can happen within hours or prevent flea infestation for an entire month.

Since fleas feed on blood, giving an oral anti flea medication to your dog is a great way to poison the fleas’ food source. There are three different medications that do just this. Nitenpyram, lufeneron and spinosad, sold under the brand names Capstar, Program, Sentinel, Trifexis and Comfortis, all kill or prevent fleas in dogs with one oral dose. Each medication, however, works a little bit differently. Understanding these differences will help you, as a dog owner, to choose the best medication for your pet.


Sold under the brand name Capstar, this medication works by interfering with nervous system activity in fleas. Once your pet swallows the medication, it passes into the animal’s bloodstream. After 30 minutes, concentration of the drug in the blood reaches an effective level. As fleas take in the medicated blood, their nervous systems shut down. This means that one tablet kills all adult fleas on your pet within a few hours of ingestion. This drug, however, only remains in the animal’s system for about 24 hours, so it is not an effective flea preventative medication. The best use for Capstar is to treat current flea infestations. You will need to use an additional product to prevent your dog from becoming reinfested with fleas.

While nitenpyram is toxic to insects, it is very safe for dogs and cats when used as directed. You can use it in puppies and kittens that are over a month old and weigh more than 4 pounds. It is also safe for use in pregnant and lactating animals.


Found in the medications Program and Sentinel, lufeneron is unique in that it targets immature fleas rather than adults. Lufeneron prevents the development of chitin in insects. This is the substance that makes up the hard exterior of an adult flea. Without chitin, immature fleas can never become adults. One dose of lufeneron remains in your pet’s system for one month, so it is an excellent flea preventative. However, the drug does not kill adult fleas. If your dog currently has fleas, you should use a drug, such as Capstar, to kill existing adult fleas. You can then use a medication containing lufeneron to prevent reinfestation. Lufeneron is safe for use in puppies and kittens older than 1 month of age, nursing females and pregnant animals.

Lufeneron is sold alone under the brand name Program for flea prevention. It is also available in combination with milbemycin oxime under the brand name Sentinel. Since milbemycin oxime prevents heartworms and treats roundworms, hookworms and whipworms, Sentinel is a comprehensive parasite control product rather than simply a flea preventative.


Contained in the medications Comfortis and Trifexis, spinosad induces muscle tremors, paralysis and death in insects. Like nitenpyram, spinosad kills adult fleas, but one dose of spinosad lasts for one month rather than just one day. This makes spinosad an effective oral flea preventative medication as well as an effective treatment for current flea infestations. The drug is safe for dogs older than 2 to 4 months that weigh more than 5 pounds. If your dog is pregnant or suffers from epilepsy, you should check with your veterinarian before using Comfortis or Trifexis. Spinosad should only be used with caution in these animals.

Like lufeneron, spinosad is sold alone or in combination with milbemycin oxime. The medication Comfortis contains spinosad as its sole active ingredient. Trifexis contains both spinosad and milbemycin oxime for broader parasite control.

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Updated: 02/23/2012, JustGreatTips
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