Proposition 65 is often called Prop 65. It is a law in the state of California that was enacted in 1986, Its full name is the Safe Drinking Water and Toxic Enforcement Act of 1986. The original purpose of this law was to help protect the citizens of California from chemicals known to cause cancer, birth defects or other reproductive harm, by limiting the amounts of these substances that can find their way into California’s drinking water, and also to inform citizens about their presence. Each year, the Governor of California publishes a list of chemicals known to have these effects.
Although Prop 65 can help citizens decrease their ingestion of toxic chemicals, the regulatory environment that has arisen linked to this law can get out of balance; in particular, as it relates to dietary supplements. For example, many plants contain naturally occurring heavy metals due to plant uptake and assimilation into the plant cell structure, and should be regulated in a different manner than environmental toxins.
Prop 65 enforces a limit that is 50 times more stringent than established FDA safety limits, making it impossible for many natural plant materials to pass these requirements.

For example, in terms of Lead:
FDA Tolerable Daily Diet Lead Intake
Children <6 yrs old 6 mcg (μg) Pregnant women 25 mcg (μg) Adults 75 mcg (μg) Prop 65 Lead 0.5 mcg (μg) Therefore, the offending lead level determined by Prop 65 is so low that many supplements based on natural whole food sources, such as sea vegetables, cannot meet this requirement, since they contain naturally occurring minerals. In order to comply with Prop 65, supplements must display a warning on the label, stating “WARNING: This product contains a chemical known to the State of California to cause cancer and birth defects or other reproductive harm.” If a product does not have this warning on the label, it cannot be sold in the state of California. This requirement has also opened the door to money hungry litigators who are aggressively suing dietary supplement manufacturers, which, in the long run, will adversely affect the consumer because it will increase the cost of doing business, and also the cost of purchasing dietary supplements.