The Office of Environmental Health Hazard Assessment (OEHHA) added glyphosate to the Proposition 65 list effective July 7, 2017, because the International Agency for Research on Cancer (IARC) found that glyphosate is an animal carcinogen and a probable human carcinogen. 

This finding was based primarily on studies of laboratory animals in which rodents exposed to glyphosate developed tumors at higher rates than rodents not exposed to glyphosate.  Proposition 65 requires that substances identified as human or animal carcinogens by IARC be added to the Proposition 65 list.

IARC is the cancer research arm of the United Nations World Health Organization (WHO).  The US government and at least 18 US states rely on IARC for its expertise in the area of carcinogen identification.  IARC convenes Working Groups of internationally renowned experts to review and evaluate the published scientific literature for chemicals, to determine whether the published data support a finding that a chemical poses a cancer hazard to humans.

In March 2015, IARC concluded that, based on sufficient evidence in animal studies, limited evidence in human studies, and other supporting mechanistic data, glyphosate is “probably carcinogenic to humans.”

Proposition 65 is a right‑to-know law that is intended to enable Californians to make informed choices about exposures to chemicals that cause cancer or reproductive effects.  Proposition 65 does not ban or restrict the use of glyphosate.  A federal court issued a preliminary injunction that prohibits enforcement of the warning requirement for glyphosate pending resolution of a challenge to the constitutionality of any warnings for glyphosate exposures.  Once the case is resolved, warnings may be required for exposures to glyphosate that exceed certain levels.

Yes, other agencies have reached different conclusions regarding the carcinogencity of glyphosate.  Proposition 65 and its related regulations rely on several agencies to evaluate cancer hazards.  Two of these agencies are currently reviewing glyphosate.  Scientific disagreement on the interpretation of data is not unusual, and does not affect the Proposition 65 listing.

The US Environmental Protection Agency has published a draft document for public comment that states that glyphosate is “not likely to be carcinogenic to humans.”

The National Toxicology Program of the US Department of Health and Human Services is undertaking additional research to investigate the potential for glyphosate to cause cancer and other possible health effects.

Other organizations, including the European Chemicals Agency's Committee for Risk Assessment, the European Food Safety Authority, and the Joint FAO/WHO Meeting on Pesticide Residues have found glyphosate is not likely to be carcinogenic to humans.