Mercury in Dog Vaccines. Should You Be Concerned?


Dog vaccines have a long history dating back to Louis Pasteur’s rabies vaccine in the 1880s. This groundbreaking vaccine was the first rabies vaccine ever created and paved the way for widespread canine vaccination programs (Vaccination of dogs). Since then, vaccines have played a crucial role in protecting dogs from deadly diseases like parvovirus, distemper, hepatitis and more. However, some controversy has emerged in recent years over the use of mercury-based preservatives like thimerosal in dog vaccines.

This article provides an in-depth look at the issue of thimerosal in dog vaccines. It examines the history and purpose of thimerosal, potential side effects, and safer alternative options. The goal is to objectively present the facts around this debate so dog owners can make informed decisions about their pet’s health.

How Dog Vaccines Work

Dog vaccines contain weakened or killed forms of disease-causing microorganisms like viruses or bacteria. They stimulate the immune system to recognize the antigen and produce antibodies against it, providing immunity without causing illness in the dog.

Here are some common dog vaccines:

  • Rabies – prevents rabies virus infection
  • Distemper – protects against distemper virus
  • Parvovirus – prevents parvovirus infection
  • Adenovirus – protects against infectious canine hepatitis
  • Parainfluenza – prevents kennel cough caused by parainfluenza virus
  • Leptospirosis – prevents leptospirosis bacterial infection
  • Bordetella – protects against Bordetella bronchiseptica bacterial infection causing kennel cough

Puppy vaccination schedules recommend starting vaccines as early as 6-8 weeks old, with boosters every 2-4 weeks until 16 weeks old. Adult dogs require vaccine boosters 1-3 years depending on vaccine type to maintain immunity.

a vet giving vaccine shots to puppies in an exam room.

The Role of Preservatives

Preservatives are added to vaccines to prevent contamination. According to the CDC, “Preservatives prevent the growth of bacteria or fungi that may be introduced from the environment during vaccine handling.”

Multi-dose vials in particular require preservatives to prevent bacterial or fungal growth between uses. As explained by the FDA, “Preservatives prevent microbial growth in the event that inadvertent contamination occurs during vaccine handling.”

The most widely used preservative in vaccines is thimerosal. According to the HHS, “Today, preservatives are usually only used in vials (small glass bottles) that contain more than 1 dose of a vaccine.” Thimerosal allows multi-dose vials to be used safely without contamination between patients.

Thimerosal Contains Mercury

Thimerosal is a mercury-based preservative that has been used in some vaccines since the 1930s. It is 49.6% mercury by weight and metabolizes into ethylmercury. This differs from methylmercury, which is the form of mercury found in certain types of fish (1).

There has been concern from some groups that the ethylmercury in thimerosal could cause mercury toxicity problems. However, ethylmercury clears from the body faster than methylmercury and is less likely to accumulate. The CDC states that ethylmercury from thimerosal does not pose a health risk except at very high exposures (2).

While the doses of ethylmercury in vaccines has not been conclusively linked to health problems, some individuals and parents still have concerns regarding exposure. This has led many vaccine manufacturers to produce thimerosal-free versions (1).




Mercury Exposure Levels

Thimerosal contains a form of mercury called ethylmercury. In dog vaccines preserved with thimerosal, the amount of mercury is very small – around 25 mcg per dose.

a dog getting a vaccine injection from a veterinarian.

Some sources show that eating 8 ounces of some fish can expose dogs to 18-166 mcg of methylmercury, a different form of mercury that is more easily absorbed. So a single fish serving may contain more mercury than an entire vaccine series.

Cumulative mercury exposures matter, but thimerosal-containing vaccines only account for a tiny fraction of most dogs’ total mercury exposure. Regular dental cleanings, fish-based foods, and environmental exposures contribute more to a dog’s overall mercury burden.

Potential Side Effects

Thimerosal contains a form of mercury called ethylmercury. High exposure to mercury can cause toxicity, and there are some concerns that the levels of mercury from thimerosal in vaccines could potentially cause side effects in pets ( However, most experts agree that the trace amounts of mercury from thimerosal in vaccines is not high enough to cause mercury poisoning or toxicity in dogs (

Potential side effects from the low levels of mercury exposure through thimerosal vaccines can include irritation at the injection site, fatigue, tremors, and neurological abnormalities. However, studies show these side effects are very rare in pets receiving thimerosal-containing vaccines (

While there are some potential risks from the mercury, the benefits of protection against deadly diseases like rabies far outweighs the low chance of side effects from thimerosal. Vaccines prevent widespread deadly outbreaks in pets. So the tiny quantities of mercury preservative are necessary for safely administering vaccines to large populations.

Thimerosal-Free Options

There are some thimerosal-free vaccine options available for dogs. According to Dog Naturally Magazine, companies like Merial make thimerosal-free rabies vaccines called IMRAB 3 TF. Rabvac 3 is another thimerosal-free rabies vaccine option.

a vet preparing a thimerosal-free vaccine syringe.

To get thimerosal-free vaccines for your dog, request them specifically from your veterinarian. Some vet offices stock the thimerosal-free versions, while others may need to order them specially. Be aware that the thimerosal-free vaccines typically cost more than versions containing thimerosal.

Downsides of the thimerosal-free vaccines are mainly just the higher cost. There do not appear to be significant drawbacks in terms of efficacy or side effects compared to thimerosal versions. However, some anti-vaxxers claim the versions with thimerosal are actually safer despite the mercury content. There is no strong evidence that the thimerosal-free vaccines have more risks or side effects.

Expert Recommendations

Major veterinary organizations, including the American Veterinary Medical Association (AVMA) and the Center for Disease Control (CDC), consider thimerosal safe to use in vaccines for dogs.

The AVMA states that thimerosal has been used as a preservative in biologics, including vaccines, without evidence of harm to animals. They point to several key factors:

– The amount of mercury exposure from thimerosal in vaccines is low. For a 20 pound dog receiving a vaccine containing 0.2 mg/ml thimerosal, the maximum exposure would be around 1/1000 of the acceptable daily intake set by the EPA (

– Ethylmercury clears the body more quickly than methylmercury and is less likely to accumulate (AVMA).

– No harmful effects have been observed from the amounts of thimerosal present in vaccines. According to the CDC, there are no clinically relevant safety risks from the thimerosal levels in vaccines.

The AVMA and CDC conclude that the low exposure levels, different form of mercury, and lack of observed harm all indicate thimerosal is safe as a preservative in vaccines for pets when used according to label guidelines.

Alternate Preservatives

Some dog vaccines use alternate preservatives instead of thimerosal. According to this WHO report, 2-phenoxyethanol is an alternate preservative used in some veterinary vaccines. It’s an organic chemical compound that has been approved for use in cosmetics and pharmaceuticals. Research indicates 2-phenoxyethanol is less toxic than thimerosal and does not pose major safety concerns at the low levels used in vaccines.
a vaccine vial with an alternate preservative label.

Another alternate preservative is benzethonium chloride, which is used in some rabies vaccines for dogs. Studies have found it to be safe and effective at preserving vaccines. While no preservative is completely without risk, these alternatives appear to provide a good option for dog vaccines with a reduced mercury level compared to thimerosal.


Thimerosal, an ethylmercury-based preservative, is present in some dog vaccines to prevent contamination. However, the mercury levels from thimerosal are significantly below toxic amounts. Extensive research shows no definitive link between thimerosal in vaccines and adverse effects in dogs. While a small subset of dogs may experience reactions, the vast majority tolerate vaccinations well. Leading health organizations and veterinary experts conclude that the benefits of protecting dogs from dangerous diseases vastly outweigh any potential minimal risks from thimerosal. Dog owners with concerns should have an open discussion with their veterinarian about vaccine options, such as thimerosal-free versions or an alternative schedule, to make the best health decisions for their pets.

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