Why do some supplements carry a Prop 65 warning?

A Prop 65 warning on a supplement means the maker or seller believes the product can expose you to at least one chemical on California's Proposition 65 list, and that the exposure may be above the level that law treats as safe. Lead, cadmium and other heavy metals are common reasons. The warning does not say how much is in the product, so it is a prompt to ask questions, not a verdict.

This guide explains what the law requires, what the warning text means, which numbers California uses for lead and cadmium, what recent studies found in supplement samples, and how to check a product before you buy. It is general information, not medical advice.

What is Proposition 65 and why does it apply to supplements?

Proposition 65 is California's Safe Drinking Water and Toxic Enforcement Act of 1986. Section 25249.6 of the state Health and Safety Code says that no person in the course of doing business shall knowingly and intentionally expose any individual to a chemical known to the state to cause cancer or reproductive toxicity without first giving clear and reasonable warning.

It is a labeling law, not a ban. Because it reaches products sold to California consumers and most online brands ship nationwide, a brand may print the California warning on every bottle, which is one reason you can see it on a product bought in another state.

The law has a built-in threshold. Section 25249.10 says the warning requirement does not apply when the business can show that the exposure poses no significant risk assuming lifetime exposure at that level, for cancer-causing chemicals, or that it will have no observable effect at 1,000 times the level in question, for reproductive toxicants. So a missing warning can mean the product tested under the threshold, or it can mean nobody checked.

What does the Prop 65 warning on a supplement actually say?

California's regulations spell out the wording for food exposures, and they include dietary supplements by name. A long-form warning begins with "WARNING:" in bold capital letters. For a listed carcinogen it continues: "Consuming this product can expose you to chemicals including" the named chemical, "which is known to the State of California to cause cancer," followed by a web address for more information. For a reproductive toxicant the last words are "birth defects or other reproductive harm." A shorter version is allowed on small labels, such as "Can expose you to" a named chemical.

Notice what is missing. The warning does not give an amount, does not say which batch was tested, and does not say how often you would have to take the product for the exposure to matter. It also does not say the product is unsafe under federal rules. It says the seller could not, or chose not to, show that the exposure is under California's thresholds.

What are the California limits for lead and cadmium?

California's regulations set numbers for some chemicals. In section 25805, which lists levels for reproductive toxicants, lead is set at 0.5 micrograms per day and cadmium (oral exposure) at 4.1 micrograms per day. The regulation says exposure at or below those levels is treated as having no observable effect, assuming exposure at one thousand times that level.

To put a microgram in perspective, a microgram is one millionth of a gram. A single capsule weighs hundreds of thousands of micrograms, so a trace of metal that is invisible and tasteless can still add up to a number near these limits across a daily serving.

Two cautions apply. First, these are California's legal levels for deciding whether a warning is needed, not medical advice about what is safe for you. Second, the daily figure is about total exposure from a product at its labeled serving, so a lab result in parts per million only becomes meaningful once it is multiplied by the amount you actually take each day. That is why a Certificate of Analysis that lists only a concentration can be hard to interpret without the serving size.

How common are heavy metals in supplements?

It depends on the product type, and the published data show that results vary by batch and by brand.

In a 2026 study in the Journal of Applied Toxicology, researchers measured arsenic, cadmium and lead in 11 batches of encapsulated green tea supplements from six brands sold in Brazil. Estimated daily exposure to arsenic (conservatively treated as inorganic) and cadmium stayed below the permitted daily exposure values in every batch. Lead, however, exceeded the USP limit for elemental impurities in two of the batches. The authors conclude that supplements can be a concentrated source of elemental exposure and that results need to be judged per daily dose, not per gram.

A 2026 study in Environmental Research tested 30 sports protein supplements sold in China and found that every sample contained at least one of the four target metals (arsenic, cadmium, lead, mercury). Plant-derived products had higher average lead (0.18 mg/kg) and cadmium (0.05 mg/kg) than dairy-derived ones (0.06 and 0.01 mg/kg), and one brown rice and cocoa product was an outlier at 0.60 mg/kg lead. The authors' risk modeling gave a hazard index of 0.45 at 30 g a day and 0.90 at 90 g a day (below 1), and 1.28 at the 95th percentile under high-use conditions.

Two lessons follow. Detecting a metal is not the same as exceeding a limit, because most foods contain traces. And the ingredient type matters: plant-based powders, in these samples, carried more lead and cadmium than dairy-based ones. That does not mean any particular brand has a problem. It means the question is fair to ask of any plant-based product, including greens powders such as the one in our Man Greens review.

What do sales pages say about heavy metals?

Heavy metals show up in supplement marketing in two very different ways, and it helps to tell them apart.

The first is the "detox" story. The Pineal XT review notes that the page says its chlorella powder binds and removes heavy metals from the pineal gland, and that we found no cited evidence for that claim. The Joint Glide review describes a sales narrative that presents heavy metals as the hidden cause of joint pain, and says that framing simplifies a much more complicated picture. In both cases the review is about a claim made on the page, not about metals being found in the product.

The second is the quiet gap. The RhythmONE review notes that shilajit, one of its ingredients, should come from a source that tests for heavy metals, and that the sales page does not address this. That is not an accusation of contamination. It is a reminder that a page can talk at length about benefits and never say a word about testing.

A fair rule: a page that promises to remove metals from your body should show evidence for it, and a page that sells a plant extract should say who tests it for metals and how to see the result.

How can you check a supplement before you buy?

1. Look for a Prop 65 statement on the bottle or product page. Its presence or absence is information, not a safety rating.
2. Ask the brand for a recent batch report that lists lead, cadmium, arsenic and mercury with limits and measured values, and check that the lot number matches your bottle.
3. Multiply any concentration by your daily serving size to estimate the daily amount, and compare it with the stated limit.
4. Be more careful with products you take daily, in large servings, for months, such as powders and greens blends.
5. Treat a "detox" or "removes heavy metals" claim as something that needs evidence on the page.
6. Talk to a doctor or pharmacist before use if you are pregnant, planning a pregnancy, or giving a product to a child.

Our how we review supplements page explains how we weigh label disclosure, and you can compare products in our supplement reviews.

FAQ

Does a Prop 65 warning mean a supplement is dangerous?

Not by itself. It means the seller says the product can expose you to a listed chemical and has not shown the exposure is under California's thresholds. The wording does not state the amount, so ask for batch test results if you want specifics.

Why do supplements sold outside California carry the warning?

The law reaches products sold to California consumers, and one label is simpler than separate packaging for each state. A brand may also add it as a precaution without having measured anything.

What is the Prop 65 limit for lead?

California's regulation lists lead at 0.5 micrograms per day for reproductive toxicity, with cadmium at 4.1 micrograms per day for oral exposure. These are legal levels used to decide whether a warning is needed, not personal medical advice.

Do plant-based supplements contain more heavy metals?

In one 2026 study of 30 protein supplements, plant-derived products had higher average lead and cadmium than dairy-derived ones. Results vary by product and batch, so a test report for the exact lot is the best check.