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Glossary

What does a Prop 65 warning mean on a supplement?

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

A Prop 65 warning is a notice that California's Proposition 65 requires when a product exposes people to a chemical on the state's list. A business does not need one if it can show the daily exposure is below a safe harbor level, such as 0.5 µg of lead. Supplements follow the state's food warning rules.1,2

Key facts

  • California's rules count dietary supplements as food, so a warning on a supplement label must use the food format and be set off in a box.1
  • Exposure is worked out by multiplying the amount of the chemical in the product by how much an average user takes.1
  • Safe harbor levels by mouth include 0.5 µg a day for lead, 4.1 µg for cadmium and 10 µg for inorganic arsenic.2
  • Reproductive safe harbor levels are set at one thousandth of the highest dose with no observable effect.1
  • Businesses with nine or fewer employees are exempt from the warning requirement.1

What Proposition 65 is

Proposition 65 is the Safe Drinking Water and Toxic Enforcement Act of 1986, run by California's Office of Environmental Health Hazard Assessment (OEHHA). OEHHA publishes the Proposition 65 list of chemicals, updated at least once a year, and a business must warn before knowingly exposing people to them unless an exemption applies.1

Enforcement is by civil lawsuit, brought by state or local prosecutors or, after notice, by private parties. Penalties reach $2,500 a day for each violation.1

What the list covers and what the warning says

Chemicals go on the list if they are known to cause cancer, birth defects or other reproductive harm. For food, including supplements, a safe harbor warning starts with WARNING: or CA WARNING: in bold capitals, then says that consuming the product can expose you to a named chemical known to the State of California to cause cancer, or birth defects or other reproductive harm, or both.1

It ends: For more information go to www.P65Warnings.ca.gov/food. The warning does not give the amount of the chemical or how far it is above the safe harbor.1

Lead is listed both as a cause of cancer and of developmental and reproductive harm. Cadmium, inorganic arsenic compounds, mercury compounds and methylmercury are on the list too.1

How a business decides whether to warn

The business works out the daily exposure: the amount of the chemical per gram of product times the amount an average user takes. If that is below the safe harbor level, no warning is needed for that chemical.1,2

Proposition 65 safe harbor levels for metals by mouth1,2
ChemicalLevel per dayKind of level
Lead0.5 µgMaximum allowable dose level
Lead15 µgNo significant risk level
Cadmium4.1 µgMaximum allowable dose level
Inorganic arsenic10 µgNo significant risk level
MercuryNone listedListed chemical with no safe harbor level

Businesses may also use other levels they can show are scientifically valid. Chemicals that occur naturally in a food do not count, to the extent the business can show they did not come from human activity and good practice could not have avoided them.1,2

What a warning does and does not tell you

The warning itself gives no amount, so it cannot tell you how much of the chemical is present. The useful next step is to ask the company for the measured amount per daily serving and compare it with the safe harbor.1

Proposition 65 is California law, separate from FDA. FDA's benchmarks for lead from food are its interim reference levels of 2.2 µg daily for children and 8.8 µg daily for females of childbearing age.3

Questions people ask

Why do so many vitamins have a Prop 65 warning?

Ingredients from plants and minerals can carry traces of listed metals such as lead, and the lead safe harbor of 0.5 µg a day is low. A product with a large daily serving can pass it even when the concentration is small.1,2,3

Should I take a Prop 65 warning seriously?

Use it as a prompt to ask questions. The warning gives no amount, so ask the company for the measured level per daily serving and compare it with the safe harbor and with FDA's benchmarks.1,3

Does a product without a warning contain no listed chemicals?

Not necessarily. It may contain a listed chemical below the safe harbor level, or the business may be exempt, for example because it has nine or fewer employees.1,2

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Sources

  1. Office of Environmental Health Hazard Assessment. Regulations on Proposition 65, California Code of Regulations, Title 27, Division 4, Chapter 1, with Appendix A, Proposition 65: A Summary. Unofficial copy, updated February 2025. Opened 2026-10-07.
  2. Office of Environmental Health Hazard Assessment. Proposition 65 No Significant Risk Levels (NSRLs) for Carcinogens and Maximum Allowable Dose Levels (MADLs) for Chemicals Causing Reproductive Toxicity. March 2021. Opened 2026-10-07.
  3. U.S. Food and Drug Administration. Lead in Food and Foodwares. Opened 2026-10-07.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.