The Truth About Oral Flea, Tick, and Heartworm Preventatives: What Every Pet Owner Should Know

As pet owners, we all want to protect our furry companions from harm, but when it comes to flea, tick, and heartworm prevention, the options can feel overwhelming. 

Among the most popular choices are oral medications, often praised for their convenience and effectiveness. But what’s actually in those tiny chewables? 

We’ve taken a clear-eyed look at oral preventatives, what they do, how they work, and the trade-offs that every pet parent deserves to understand. 

What are Oral Flea, Tick, and Heartworm Preventatives?

Oral flea, tick, and heartworm medications are chewable or pill-form treatments designed to protect pets from parasites. Veterinarians commonly prescribe them and often market them as convenient, long-lasting, and highly effective. 

These products are typically administered monthly and aim to either kill parasites on contact or prevent the life cycle of heartworms. While they can help reduce infestations, it's essential to understand what these medications do inside of your pet’s body.

How These Medications Work

These preventatives work systemically. Once your pet swallows the medication, the active ingredients circulate through their bloodstream. 

When a flea, tick, or mosquito bites your pet, it ingests the medication and is killed shortly after. For heartworms, the medications work by eliminating the larval stage of the parasite before it can mature and cause severe damage.

While this might sound ideal, the mechanism of action raises questions about safety. 

Why “Kill After Bite” Isn’t Truly Preventative

The term "preventative" suggests protection before a problem begins. However, oral flea and tick medications only start working after the parasite bites. This means your pet is not actually shielded from exposure to ticks or fleas; it’s just that the pest dies afterward.

This still allows for disease transmission. For example, a tick can transmit Lyme disease within hours of attachment. If the tick dies after biting, but not before it has transferred pathogens, your pet remains at risk. Proper prevention would involve repelling the parasite altogether, which oral medications do not do.

4 Common Active Ingredients in Preventatives and Their Risks

1. Ivermectin

Commonly used in heartworm medications, ivermectin is an antiparasitic. 

While effective, it can deplete essential vitamins. Prolonged or repeated use of ivermectin-containing medications may result in deficiency symptoms like fatigue, skin issues, cognitive decline, and increased vulnerability to illness, especially in dogs with pre-existing conditions or poor diets.

2. Isoxazolines (e.g., Afoxolaner, Fluralaner, and Lotilaner)

These compounds are often found in flea and tick preventatives. They work by disrupting nerve signaling in parasites. They can also affect pets’ nervous systems and have been linked to tremors, seizures, and neurological issues, even in pets without a history of such problems.

3. Spinosad

This ingredient targets the nervous system of insects. Though it may be effective, spinosad has been associated with vomiting and lethargy in pets.

4. Milbemycin Oxime

Used to prevent heartworms and intestinal parasites, this chemical can sometimes lead to digestive issues or more serious neurological effects, especially in sensitive breeds.

Why They’re Classified as Neurotoxins

Many active ingredients in oral parasite medications are classified as neurotoxins because they target the nervous systems of fleas, ticks, or worms. The issue is that these chemicals don't always distinguish perfectly between parasite and pet. 

They can cross the blood-brain barrier or interfere with neurological function in dogs and cats, especially if a pet has a genetic sensitivity, such as the MDR1 mutation. It poses a seizure risk in dogs with the MDR1 gene mutation, which affects how certain drugs are processed in the brain.

The FDA has even issued warnings about isoxazoline-based products, urging pet owners and veterinarians to be aware of potential adverse effects.

2 Other Risks of Oral Preventatives

Medications Remain in the Body for Over 30 Days

Unlike topical treatments that can be washed off or physically removed, oral preventatives stay in your pet’s bloodstream for weeks, often over 30 days. This extended presence means that if your pet experiences a bad reaction, the effects may persist or escalate. 

You cannot simply “undo” the dose once it’s been ingested. This long duration also increases the cumulative burden on your pet’s liver and kidneys, which are responsible for processing and eliminating the chemicals.

Nutrient Depletion Caused by Ingredients Like Ivermectin

Ivermectin, a common ingredient in heartworm preventatives, has been linked to depletion of critical B vitamins (B1, B2, B3, B6, B7, B9, B12) as well as vitamin K. These nutrients are essential for energy metabolism, nerve function, and blood clotting. 

3 Holistic Alternatives to Chemical Preventatives

More pet owners are turning away from chemical-based flea, tick, and heartworm preventatives in favor of natural approaches that support long-term wellness. 

Holistic strategies aim to make your pet less appealing to parasites, support internal health, and maintain a clean, pest-free environment. These methods are especially beneficial for sensitive dogs or those with underlying health issues.

Natural Flea and Tick Repellents

Essential oils like cedarwood, lemongrass, and neem are commonly used in pet-safe sprays, shampoos, and collars. One particularly effective internal solution is Vitamin B-Complex supplementation.

Our Vitamin B-Complex is uniquely designed to mimic the ratios found in natural liver protein. It includes an enhanced level of vitamin B1 (thiamine), which has been shown to repel fleas and ticks naturally.

B vitamins support digestion, reduce anxiety, improve oral health, minimize shedding, and help manage weight and stool-eating behaviors. This approach focuses on overall vitality, not just pest deterrence.

Strengthening Immunity to Ward Off Parasites

A strong immune system is your pet’s first line of defense against parasites. When the immune system is functioning properly, it can help prevent infestations from taking hold. Digestive enzymes play a vital role in maintaining immune health by supporting gut function.

These enzymes break down food more efficiently, allowing for better nutrient absorption and reducing strain on the digestive system. Volhard’s formula includes prebiotics and probiotics along with pancreatin, a powerful enzyme that helps the pancreas function optimally. A healthy gut means fewer toxins, better elimination, and improved overall immunity, which can help reduce the chances of your dog becoming a host to parasites.

Environmental Management and Clean Living

Even the best internal support won’t work if your pet’s environment is loaded with pests. Environmental management is key in any holistic parasite prevention plan. This includes:

• Frequent vacuuming and washing of pet bedding.
• Use of food-grade diatomaceous earth around the home and yard.
• Regular grooming and skin checks.
• Avoiding pesticide-treated grasses and communal dog areas during peak parasite seasons.

A clean, toxin-free home supports your pet's health and minimizes exposure to external stressors. Combined with nutritional and immune support, these practices can create a holistic barrier that keeps parasites at bay, naturally and effectively.

Choose Natural Protection: Support Your Dog’s Health With Volhard Dog Nutrition!

For more advice on dog nutrition, health, and training, contact us and check out our other blogs, too!

Volhard Dog Nutrition and its expert canine nutrition coaches offer online consultations to help more dog parents discover why and how to feed their dogs the healthiest foods!

Speaking to a Volhard canine nutrition coach will help you understand the inseparable relationship between healthy food, a healthy body, and a healthy mind.

If you want to contact one of our Volhard canine nutrition coaches, don't hesitate to access our consultation page!

References:

1. "Neurologic Event Potential Associated with Certain Flea/Tick Products." U.S. Food and Drug Administration, FDA, www.fda.gov/animal-veterinary/cvm-updates/animal-drug-safety-communication-fda-alerts-pet-owners-and-veterinarians-about-potential-neurologic. Accessed 23 Apr. 2025.
   
   
2. Dryden, Michael Wayne. "Flea and tick control in the 21st Century: Challenges and opportunities." Veterinary Dermatology, vol. 20, no. 5–6, Oct. 2009, pp. 435–440, https://doi.org/10.1111/j.1365-3164.2009.00838.x. Accessed 23 Apr. 2025.