Are Cancer Causing Ingredients in Pet Shampoo? What to Avoid to Protect Your Dog

As we continue our month long focus on pet cancer awareness, it is important to understand how an ingredient is classified as "cancer causing". 

It is a LONG road to get on that list!

So, who decides, how evidence is evaluated, and what do those classifications actually mean?

Scientists and public-health agencies use tiered systems to assess whether a substance can cause cancer and under what conditions exposure might matter.

The Major Scientific Organizations Who Classify Carcinogens

• International Agency for Research on Cancer (IARC): Part of the World Health Organization (WHO). IARC reviews global data from animal studies, human epidemiology, and mechanistic evidence to classify a substance as hazardous - that is, whether it can cause cancer under some circumstances.

  • Group 1: carcinogenic to humans (strongest evidence)
  • Group 2A: probably carcinogenic
  • Group 2B: possibly carcinogenic
  • Group 3: not classifiable (evidence insufficient/uncertain)
    
    

• U.S. National Toxicology Program (NTP): Run by NIH, CDC, and FDA. NTP's Report on Carcinogens lists substances that are "known" or "reasonably anticipated" to be human carcinogens, based on laboratory and epidemiological data.

• U.S. Environmental Protection Agency (EPA): Evaluates carcinogenic risk by combining hazard classification with exposure estimates to decide if real-world use levels are likely to pose harm.

• European Union's Scientific Committee on Consumer Safety (SCCS): Advises the European Commission on cosmetic ingredient safety and sets concentration limits or bans for hazardous substances.

• Food and Drug Administration (FDA): Oversees cosmetics (including pet grooming products in many jurisdictions), ensuring products are not "adulterated" or "misbranded." FDA often defers to NTP, IARC, or EPA hazard findings when setting guidance.

• California's Office of Environmental Health Hazard Assessment (OEHHA): Maintains the Proposition 65 list, requiring consumer warnings for chemicals "known to the State of California to cause cancer or reproductive toxicity."

What This Means for Pet Shampoos

Pet grooming products occupy an unusual regulatory space.

In the United States, they're generally classified as consumer products or "grooming aids"- unless they make therapeutic claims such as killing fleas or treating skin disease. That distinction is important because it determines who regulates the product and how closely it is regulated.

Unlike prescription veterinary drugs, pet shampoos do not require pre-market approval by the Food and Drug Administration (FDA). Instead, manufacturers are simply expected to ensure their products are safe for their intended use and are properly labeled.

A pretty big ask when many manufacturers are solely focused on making money, right?

Oversight falls very loosely under the Consumer Product Safety Commission (CPSC) framework established in the 1970s, last updated through the Consumer Product Safety Improvement Act of 2008, which focused primarily on children's products, not pet care.

By contrast, human cosmetics just underwent their first major regulatory overhaul in more than 80 years with the Modernization of Cosmetics Regulation Act (MoCRA), signed into law in December 2022. MoCRA now requires cosmetic manufacturers to register facilities, list ingredients, report adverse events, and gives FDA authority to issue mandatory recalls - steps that significantly increase transparency and consumer protection. It still doesn't protect against contaminants, but it is certainly better than it was 80 years ago.

Unfortunately, MoCRA does not cover pet shampoos.

That means there is currently no mandatory federal system in the United States ensuring pre-market ingredient review, impurity testing, or carcinogen monitoring for pet grooming products.  Read that again!

Unless a manufacturer voluntarily follows MoCRA-level standards, pet shampoos remain subject only to general safety obligations under the consumer-product umbrella.

This regulatory gap underscores why it's essential for both pet parents and responsible brands (like 4-Legger) to take the initiative.

With limited formal oversight, the best safeguard is your own ingredient review - relying on the global carcinogenicity assessments conducted by agencies such as IARC, NTP, EPA, and the EU SCCS when deciding which preservatives, surfactants, and fragrances to use or avoid.

In short: While the law hasn't fully caught up, informed consumers and conscientious brands can lead the way by choosing transparency, third-party ingredient vetting, and safer formulations long before they're required by regulation.

Ingredients (and Impurities) of Concern in Pet Shampoos

1. Formaldehyde & formaldehyde-releasing preservatives (FRPs)

Why they're on the radar: Formaldehyde is an IARC Group 1 human carcinogen (linked to nasopharyngeal cancer and leukemia). Certain preservatives release small amounts of formaldehyde over a product's shelf life to control microbes (e.g., DMDM hydantoin, imidazolidinyl urea, diazolidinyl urea, quaternium-15, polyoxymethylene urea, sodium hydroxymethylglycinate). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans Volume 88

Regulatory momentum: The EU now requires "releases formaldehyde" warning labels for products with releasers above very low thresholds; non-compliant products had 2024-2026 phase-out deadlines. The U.S. also notified a proposed restriction on intentional addition of formaldehyde/FRPs in cosmetics (target effective date Jan 1, 2027, if finalized). Several U.S. states (e.g., WA) are also acting. Critical Catalyst EU Deadline on Formaldehyde Disclosure 2024

Bottom line: Because the parent chemical is a Group 1 carcinogen and FRPs are designed to release it, avoid FRPs in pet shampoo whenever possible. Look for these names on labels: DMDM hydantoin, imidazolidinyl urea, diazolidinyl urea, quaternium-15, polyoxymethylene urea, sodium hydroxymethylglycinate.

2. 1,4-Dioxane (contaminant in some surfactants)

What it is: A solvent impurity that can appear in ethoxylated ingredients (e.g., "-eth" or PEG-: sodium laureth sulfate, PEG-80, polysorbates) if manufacturing isn't tightly controlled.

Cancer status: Likely to be carcinogenic to humans (U.S. EPA IRIS) / possibly carcinogenic (IARC); HHS lists it as reasonably anticipated to be a human carcinogen. US EPA Dioxane Snapshot

Regulatory signal: FDA has issued manufacturer guidance to minimize nitrosamines and 1,4-dioxane contamination in cosmetics; oversight continues. U.S. Food and Drug Administration

Bottom line: Choose shampoos that avoid ethoxylated surfactants or come from brands that specifically test for and avoid 1,4-dioxane.

3. Diethanolamine (DEA) & DEA-based ingredients (e.g., cocamide DEA)

What they are: Foam boosters formed by reacting fatty acids with DEA (e.g., cocamide DEA). Cancer status: Cocamide DEA is IARC Group 2B (possibly carcinogenic) based on dermal animal studies; California lists it under Prop 65. The NTP found topical DEA exposure caused tumors in lab animals; FDA notes the study did not establish a human link, but residual DEA in related ingredients is the concern. NCBI

Bonus risk: DEA/TEA/MEA can form nitrosamines (some are potent carcinogens) if they co-exist with nitrosating agents or nitrate contamination during manufacturing. FDA and CIR have flagged this formation risk for decades. U.S. Food and Drug Administration

Bottom line: Avoid cocamide DEA and limit ethanolamine-based ingredients (DEA/TEA/MEA) in pet shampoos-especially when "nitro-/nitrite-" ingredients or contamination are possible.

4. Coal tar & coal-tar-derived actives

What it is: A complex mixture used historically in "tar" shampoos (mainly human OTC dandruff/psoriasis).

Cancer status: Coal tars and coal-tar pitches are IARC Group 1 (carcinogenic to humans). Dermatology literature acknowledges skin-cancer signals from long, concentrated exposures, while U.S. OTC monographs still list coal tar as "Category I (safe and effective)" for certain human uses. For pets, safer alternatives exist so avoid tar products unless a veterinarian specifically prescribes and explains the risk/benefit. NCBI

Bottom line: Skip tar-based pet shampoos. If medically indicated, use under veterinary direction and keep exposure minimal.

5. Phthalates (often hidden inside "fragrance")

Where they show up: As solvents/fixatives in fragrance (historically DEP, DBP, sometimes DEHP). Cancer/health status: DEHP has sufficient animal evidence and is reasonably anticipated to be a human carcinogen (U.S. HHS), while IARC currently lists DEHP as Group 3 (not classifiable) for humans, reflecting uncertainty in direct human data.

The EU has banned or severely restricted several phthalates in cosmetics; DEP remains under debate but is allowed in cosmetics within the limits set by EU regulations. 15th Report on Carcinogens

Bottom line: Because phthalates are also endocrine-active and commonly unnecessary in rinse-off pet shampoo, choose "fragrance-free" or phthalate-free formulas and avoid vague "parfum/fragrance" when possible (or seek full disclosure from the brand).

Practical label checklist for pet parents

Avoid when you see (especially together):

• Formaldehyde or FRPs: DMDM hydantoin, (dia/imid)azolidinyl urea, quaternium-15, polyoxymethylene urea, sodium hydroxymethylglycinate.

• DEA/TEA/MEA (and cocamide DEA).

• Ethoxylated surfactants (PEG-, "-eth" like laureth) without brand assurances on 1,4-dioxane control/testing.

• Coal tar / "tar" actives in pet shampoos (unless vet-directed).

• Vague "fragrance/parfum" (possible phthalates) unless the brand discloses "phthalate-free."

The Bottom Line

Some ingredients used in pet shampoos have documented scientific links to causing cancer and are classified by the agencies that do the research to identify ingredients as dangerous. 

As a responsible pet parent, it's important to know what's in your dog's shampoo.

Take a few minutes to look up every ingredient in a trusted source like the Pet Shampoo Ingredient Database to understand each ingredient's safety profile and regulatory status.

Choosing products made with non-toxic, transparently sourced, and evidence-based ingredients, like those used by 4-Legger.

This is one of the simplest and most powerful ways to reduce your dog's exposure to harmful chemicals and support long-term health.