More Questions About Sentinel Flea and Heartworm Medication for Dogs

I’ve received quite a few more questions about Sentinel for dogs so I thought I would address these in another post. A lot of people seem to be confused about what other medications can be given along with Sentinel as well as the fact that Sentinel is both a heartworm and a flea medication. Also, it’s important to remind everyone that even though Sentinel prevents flea reproduction, it does not kill adult fleas! :-)

Sentinel Flavor Tabs for DogsIf I don’t address your question or concern below please leave me a note in the comments section below. Also, you might find the information you need in my earlier post about Sentinel.

1. Does My Dog Really Need a Heartworm Test Before Giving Sentinel?

The active ingredient in Sentinel for the prevention of heartworm infection is milbemycin oxime. Milbemycin oxime works a little differently than other ivermectin-based heartworm products including Heartgard and Heartgard Plus. If milbemycin oxime is given to a heartworm-positive dog, the microfilariae are killed much faster than with the ivermectin products. This puts the dog at greater risk for a shock-like reaction if many larvae die at once. Of course, a heavily infected dog will be more at risk than a dog with a light infection.

For liability and safety reasons, veterinarians are required to use medications according to their labeled instructions. The Sentinel product insert states:

Prior to administration of Sentinel Flavor Tabs, dogs should be tested for existing heartworm infections. Infected dogs should be treated to remove adult heartworms and microfilariae prior to intitiating treatment with Sentinel Flavor Tabs. Mild, transient hypersensitivity reactions manifested as labored respiration, vomiting, salivation and lethargy have been noted in some treated dogs carrying a high number of circulating microfilariae. These reactions are presumably caused by release of protein from dead or dying microfilariae.

If you do not wish to test your dog for heartworm infection prior to giving him Sentinel your veterinarian may or may not allow you to sign a medical waiver.

2. Can I Get Sentinel Flea Pills for Dogs Without a Prescription?

Because Sentinel is also a heartworm medication there is no way to get it without a prescription. However, if you only need the flea preventative, lufenuron, this can be purchased without a prescription from your veterinarian. You should look for a product called Program, which contains the medication lufenuron and nothing else. Remember that lufenuron only inhibits the development of flea eggs and does not kill adult fleas. If you find someone selling Sentinel without a prescription online it’s probably not worth buying.

3. What’s the Difference Between Interceptor and Sentinel for Dogs?

Interceptor only contains the active ingredient milbemycin oxime which is used as a preventative for heartworm infection as well as intestinal parasites such as roundworms, hookworms, and whipworms. Sentinel contains milbemycin oxime as well as an additional ingredient, lufenuron, which “sterilizes” fleas. Because it contains an additional medication, Sentinel is more expensive than Interceptor.

4. What ingredients does Sentinel contain?

As stated previously, there are two Sentinel ingredients: milbemycin oxime and lufenuron. Milbemycin oxime is used for the prevention of heartworm infection as well as the control of numerous intestinal parasites including roundworms, hookworms, and whipworms. Lufenuron inhibits the development of flea eggs.

5. How Does Sentinel Work?

You can find this information by reading the product insert:

Milbemycin oxime, one active ingredient in Sentinel Flavor Tabs, is a macrocyclic antihelmintic which is believed to act by interfering with invertebrate neurotransmission. Milbemycin oxime eliminates the tissue stage of heartworm larvae and the adult stage of hookworm (Ancylostoma caninum), roundworm (Toxocara canis and Toxocara leonina) and whipworm (Trichuris vulpis) infestations when adminstered orally according to the recommended dosage schedule.

Lufenuron, the other active ingredient in Sentinel Flavor Tabs, is an insect development inhibitor which breaks the flea life cycle by inhibiting egg development. Lufenuron’s mode of action is interference with chitin synthesis, polymerization and deposition. Lufenuron has no effect on the adult flea. After biting a lufenuron-treated dog, the female flea ingests a blood meal containing lufenuron which is subsequently deposited in her eggs. Lufenuron prevents most flea eggs from hatching or maturing into adults and thus prevents and controls flea populations by breaking the life cycle.

Some important information to note is that the active ingredients in Sentinel work systemically. That is, the ingredients are absorbed into the bloodstream. This means that in order for lufenuron to work, the fleas have to bite your dog. So, if you know your dog has flea allergies (flea allergy dermatitis) you might want to consider using a different flea control product.

6. Where Can I Find Cheap Sentinel For Dogs?

Because Sentinel Flavor Tabs for dogs contains both a heartworm preventative and a flea medication it costs more than other products. The best way to find the cheapest price on Sentinel is to simply search online for the best price you can find. However, I don’t recommend making your final purchase on the Internet.

What I would do is ask your veterinarian if they will match some of the cheaper prices you find online. They will probably need to see a current print-out of some of the websites you visit because these prices might vary from one week to the next. That way you know you’re getting the best and safest product by purchasing from your veterinarian directly. I think as more people demand lower prices veterinarians will follow suit. Of course, veterinarians have a lot more overhead than online retailers which accounts for the higher prices. Animal hospitals also offer better customer service and know more about the products than online retailers. You might also notice there’s a lot of variability in prices from one vet to the next. However, at the end of the day, I always think it’s better to support your local veterinarian whom you’ve established a relationship with, rather than some faceless entity you stumble upon online.

7. Can I Use Sentinel With Other Flea Control Products Like Frontline?

Yes. In fact, if you see adult fleas on your dog concurrent use of an adulticide like Frontline or Capstar is both recommended and encouraged to provide adequate flea control. Dogs that have repeated exposure to flea infested animals or environments will always have adult fleas if an adulticide is not used. An adulticide is a product that kills adult fleas.

I would probably use Capstar instead of Frontline though. Mainly because there seems to be a growing problem with owner compliance in terms of applying Frontline correctly. I think the dog is more likely to get the proper dose of medication if a pill is given instead of a topical product. When it comes to creating the most effective system for flea control, I think the concurrent use of Sentinel and Capstar gives you the greatest chance of success. The only downside to Capstar is that it only lasts for 24 hours.

The only product I would hesitate giving along with Sentinel is Revolution, or any other flea control product that also contains a heartworm medication. Although this extra dose of heartworm medication may not hurt your dog, you don’t want to give him more medication than he really needs. The manufacturer states that when the active ingredients in Sentinel were used with a variety of adulticides, vaccines, antibiotics, antehelmintics (dewormers), steroids, flea collars, shampoos, and dips, no adverse reactions or interactions were noted in dogs. If you’re concerned about using Sentinel with other medications or products you should contact your veterinarian for advice.

Sentinel Product Insert
Preventing Heartworm Infection in Dogs (Chemoprophylaxis)
Veterinary Drug Handbook (Fourth Edition) by Donald C. Plumb

Amanda is a 2-Star Diamond Beachbody Coach, retired Licensed Veterinary Technician and RRCA certified running coach. Her geekiness is legendary and is most proud of the fact she finally found a way to turn her passions for running, fitness, nutrition, writing, and pets into a full-time business that now helps others get off the couch and get after their dreams. Proverbs 3:5-6


  1. says

    Just find your blog! Very insightful information on Sentinel. Thanks for helping to spread the word about the importance of heartworm prevention and flea medications!

  2. diane johnson says

    my middle dog (quick fella) managed to take a second sentinel tab when my old dog spit hers out and I could not grab it fast enough.

    Is that dangerous for him?

    I will get another tab for old dog.

  3. says

    Hi Diane, it’s probably OK if your dog received a second tablet (the amount given for heartworm prevention is actually a very low dose, much higher amounts have been given to dogs with no side effects during testing). Of course you should always check with your veterinarian first when things like this happen.

  4. Sze says

    Thanks for this blog. I am currently using heartgard + drontal allwormer + program on my 4 months old puppy. Can I switch to sentinel instead to cover the same or will I need to add another pill to cover for tapeworm? what should I use? When do I switch medication without missing on the heartworm treatment? Thanks

  5. says

    You would need another pill if tapeworm is an issue for your puppy. There’s a product called Vethical that contains ivermectin (for heartworm prevention) plus a broad-spectrum dewormer that also takes care of tapeworms. But as far as I know it is only available at VCA Animal Hospitals. I would check with your veterinarian for the best advice including when is the best time to make a transition.

  6. Kegan says

    Hi, back in January my vet did a test for heartworms and it came back negative. They just gave me a years supply of sentinel today, one tech said I’ll need to do another test before starting sentinel since it’s been more than three weeks, and another tech said I’d be okay. Do you have any thoughts?

  7. Amy says

    Hi. Our vet gave us a Sentinel pill two weeks ago for our 8.4 lb labradoodle puppy. She was 8 weeks old at the time. Yesterday we went back for puppy shots. She weighed 11.6 lbs. The vet then said to give her one Sentinel pill (for the 10-25 lb dogs) every two weeks. I gave her the new pill this morning and she’s been a little off all day. Is giving the Sentinel every two weeks to a puppy a standard practice?

  8. says

    There is medication for intestinal parasites in Sentinel. Since you have a puppy that may be why your vet is recommending a dose every two weeks. But your vet would know better than me so I would ask them.

  9. Katie says

    Hello. I have a puppy who has been away at a trainer (potty training issues) and I forgot to give him Sentinel while he was there. He is now 4 weeks behind. Do I need to get him tested for heartworm before giving him his next dose?

  10. Chris says

    Hi! I had two dogs, one was 51lbs and my other one is 44lbs. I had to put my first dog down this past week (she was 16 years old); however, I have 2 sentinel pills left from her and none for my other dog. Would it be okay to give my other dog the 51lb and over pill? The vet will not exchange or take these pills back because they are a prescription. Thanks.

  11. Denise says

    My Wiemie is on Sentinel. He brought fleas into the house. They got on my cat, quite a few. I have flea dipped my cat and given him a OTC-Hartz OneSpot topical treatment. I flea dipped my dog also. But now I see I had other choices from your blog. My dog goes in the woods and has brought back fleas the past week again. Until it snows (I’m in MI, will the snow kill the fleas outside?) I am keeping the dog on a leash. My question is what else should I do to my house other then clean, I sprayed where I think I should? And what else should I do for my cat? I am nervous he is going to get them back from some hiding place in the house where fleas may be. Will they reproduce from the cat even though he never goes out side and I know he got from the dog? Sorry so many questions. TX

  12. Alex says

    I gave my 2 year old, 15 lb Shih-tzu her first Sentinel tab. Within 1/2 day she was vomiting. To my horror a couple of the times the vomit was red with blood. I contacted my vet. They told me that stomach upset can happen but if I felt she needed a “check” I could come in. After an x-ray, IV and nausea medication they sent us home with follow-up care. It took 3 days, constant monitoring, a number of clean-ups and a $500 vet bill before this poor thing was feeling better. I am just grateful that she is recovering but I question the safety of this product. I only hope that there are no long term effects. I always read the side effects on the box but I never thought they would be this bad. Are there any alternatives to this medication? I do not want to subject my dog to this situation again.

  13. says

    Alex your veterinarian is the best person to talk to about this. There are possible side effects with any type of medication…human or pets.

  14. Kat says

    My pomahuahua is 2 yrs old and has been on Sentinel for a year or so now. Lately she’s been itching and scratching like crazy. I can’t find any fleas, or signs of a rash. Any ideas?