Glyphosate is a probable carcinogen

May 2015

As part of its remit to re-evaluate the carcinogenic potential of agri-chemicals, the International Agency for Research on Cancer (IARC) has just produced its report on glyphosate herbicide.

The outcome is that gyphosate is now classified by the World Health Organisation (WHO) body as "probably carcinogenic to humans". This is one step short of "carcinogenic", a category to which very few chemicals as assigned.

Accepted scientific conclusions require that several different lines of evidence are investigated, all of which support the conclusion drawn. Accordingly, the IARC routinely examines data from three types of study:

1. Is there evidence of cancer in animal feeding studies?
The answer to this is 'yes', there are five positive rodent experiments, four of which fed glyposate and one which exposed skin to Roundup glyphosate-based weedkiller.

Cancer is difficult to test in the laboratory because it isn't a direct reaction to a toxin but rather an unusual combination of gene-stressing or gene-altering events. Experiments need to be big enough and long enough to have any chance of picking it up, and the subjects must be susceptible: false negatives are likely.

This means that even a handful of positive results is sufficient to suggest glyphosate is a 'possible carcinogen'.

2. Is there evidence of cancer in humans populations with known exposure to glyphosate?
One out of two population studies produced evidence of an increase in Non-Hodgkins Lymphomas, a groups of blood-cell cancers. In addition, pre-cancerous conditions were observed in the blood of a further heavily glyphosate-exposed population.

That many sectors of the population are exposed to glyphosate is not in doubt (see below). In its preamble, the IARC notes the sharp increase in exposure since GM glyphosate-tolerant crops came on the market.

Because experimentation on humans is not allowed, epidemiology is one of the few sources of human evidence available, but it provides one of the weakest sources of data: there are always multiple confounding factors (such as concurrent exposure to other cancer-causing chemicals), and uncertainty about exposure level (due, for example, to compliance with safety instructions, and reliability of memory). In the case of agri-chemicals, populations known to be exposed are almost all male: sex-specific effects will not show. The long lag-time between exposure and manifestation of cancer reduces certainty, and epidemiology is insensitive to cancers were age or timing of exposure is paramount. Also, identifying a non-exposed comparator population when all walks of life are now coming into contact with glyphosate is almost impossible.

Again, false negatives are likely, and the existence of even a single positive finding is sufficient to suggest glyphosate is a 'possible' cause of cancer.

3. Is there evidence that mechanisms known to have a cancerous outcome are linked to glyphosate exposure?
This question explores mechanistic studies not available when many agri-chemicals were originally approved.

Indeed, there is robust (repeated) evidence that glyphosate induces DNA and chromosome damage which is a clear risk factor for cancer. There is also robust evidence from animal and tissue culture studies that glyphosate, it's main breakdown product (AMPA), and glyphosate-based formulations such as 'Roundup', cause oxidative stress in cells which is known to induce DNA-damaging metabolic products.

This evidence raised the stakes of glyphosate from 'possible' carcinogen to 'probable' carcinogen.

The IARC doesn't have regulatory powers. Neither does it have a commercial or political agenda to colour its conclusions. It's panel consisted of 17 experts from 11 countries who spent almost a year preparing a comprehensive review of the latest scientific evidence. The panel members were selected on the basis of their expertise and absence of real or apparent conflicts of interest. They considered "reports that have been published or accepted for publication in the openly available scientific literature as well as "data from governmental reports that are publicly available". These are important points, because previous assessments of glyphosate have hinged largely on confidential industry tests.

Testbiotech has revealed that the recent risk assessment on glyphosate carried out by Germany for the European Food Safety Authority took only two studies on oxidative stress into account and these not in relation to cancer. Between 2005 and 2013, there were at least eight further publications reporting that glyphosate can act as an oxidative stressor in vertebrates.


Odd that world-wide regulators using industry information have managed to overlook the genotoxicity and carcinogenic potential of glyphosate.

As one of the very few unbiased opinions on the science of a very important and long-term aspect of glyphosate safety, the IARC could and should be a powerful stimulus for change. However, with a $6 billion market at stake, be prepared to do quite a bit of shouting to make sure this 'probable' carcinogen is removed from your food chain.

Exposure to glyphosate.

Besides being accumulated by GM crops grown for animal feed in the Americas, glyphosate is used on wheat and other crops as a desiccant prior to harvesting in many countries, and residues collect in the soil. In Ontario, Canada, glyphosate use rose some 76% between 2003 and 2008.

Surveys in the US document widespread water contamination, and residues are showing up in food.

In the UK, glyphosate residues have been found consistently in starchy foods such as cereal bars and bread. Root vegetables such as potatoes and carrots are picking up glyphosate from the soil.


  • Carcinogenicity of tetrachlorvinphos, parathion, malathion diazinon and glyphosate, The Lancet, 20.03.15
  • IARC Monographs Volume 112: evaluation of five orgnaophosaphate insecticides and herbicides, World Health Organization International Agency forResearch on Cancer, 20.03.15
  • IARC Monographs, Volume 100,
  • Global health experts say glyphosate "probably carcinogenic" to humans, Pesticide Aciton Network North America, 23.03.15
  • Dr. Mae-Wan Ho, Glyphosate 'Probably Carcinogenic to Humans' Latest WHO Assessment, Institute of Science in Society Report 24.03.15
  • Carey Gillam, Scientist defends WHO group report lining herbicide to cancer, Reuters 26.03.15
  • Michelle Adelman, Herbicide horror,, 1.04.15
  • Cydney Hargis, U.S. weighing increase in herbicide levels in food supply, IPS News, 2.05.13
  • Glyphosate is "probable human carcinogen" - WHO's cancer agency, GM Watch, 20.03.15
  • Does glyphosate cause cancer: Important gap in German risk assessment, Testbiotech, 15.04.15
  • A. J. De Roos, et al., 2005, Cancer Incidence among Glyphosate-Exposed Pesticide Applicators in the Agricultural Health Study, Environmental Health Perspectives

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