Proposition 65 (Prop 65), formally titled the Safe Drinking Water and Toxic Enforcement Act of 1986, requires California businesses to provide clear and reasonable warnings to consumers before knowingly and intentionally exposing them to chemicals listed by the state as the cause of cancer, birth defects, or other reproductive harm.

This legislation requires the state to maintain and update a list of chemicals known to the state to cause cancer or reproductive toxicity. The Prop 65 list, first published in 1987 and updated at least once a year, now includes around 900 chemicals. The list contains a wide range of naturally occurring and synthetic chemicals that include additives or ingredients in pesticides, common household products, food, drugs, dyes, or solvents. Listed chemicals may also be used in manufacturing and construction, or they may be byproducts of chemical processes, such as motor vehicle exhaust.

Violations of Proposition 65, stemming from the failure to provide warnings, can result in penalties reaching up to $2,500 per violation per day.