Mar 16 2017

Does Glyphosate Cause Cancer?

glyphosate-effects-fbGlyphosate, the active ingredient in Roundup, is the most popular herbicide used in the world. It has gained particular attention because several of the more commonly used GMOs are glyphosate tolerant, and therefore are intended to be used with the herbicide. Glyphosate is also manufactured by Monsanto (although it is off patent and there are generic versions available).

The question of the safety of glyphosate is in the news again after the New York Times did an article about a recent court case against Monsanto and the documents revealed through discovery and made public by the judge. Unfortunately, in my opinion the NYTs article is poorly done, and reveals significant bias – anti biotech bias is nothing new for the NYTs or the author of this article, Danny Hakim.

Last year I wrote about another article that Hakim wrote in the NYTs about GMOs, concluding:

In my opinion Hakim’s article in the Times was a hack piece with a biased narrative that is nothing more than a rehash of tired anti-GMO tropes that have already been widely deconstructed. He is entering this conversation late, and isn’t up to speed.

There are essentially two questions raised by Hakim’s latest article. The first concerns the behavior of Monsanto. Hakim alleges that they ghostwrote scientific articles for academics and used political pressure to shut down EPA reviews of glyphosate’s safety. I would not assume this assessment is true, and certainly don’t trust Hakim’s journalism given his history. The academics in question deny the allegations, and Monsanto claims these e-mails are taken out of context. We have certainly seen that before.

I don’t have enough info at this time to form my own opinion, but let’s assume that the worst interpretation of the allegations are true. This means that Monsanto tried to put its thumb on the scale to garner favorable reviews for its products. If true, this is clearly wrong. Corporations can (and often must) fund research, but they need to let the chips fall where they may. Academics should resist the temptation to accept too much “help” from corporations. This behavior is, unfortunately, very common and not limited to Monsanto.

But these allegations need to be kept in proper context. Not to excuse them in any way, but Hakim implies that this behavior raises fresh concerns about the safety of glyphosate, which is the second core question raised by the article. I don’t think that it does, however. There is nothing in the revealed e-mails that calls into question the large body of independent scientific research into the safety of glyphosate, any more than the infamous e-mails called into question the large body of scientific research establishing man-made global warming.

Let’s now do something which Hakim did not do in his article – take a look at the actual scientific evidence and see what it says.

When researching the safety of a substance there are several kinds of data. In vitro data looks at the substance in question when exposed to cells in culture. This kind of research is for plausibility only. Because the cells are being directly exposed, which is not analogous to what happens in a living organism, you cannot draw any final conclusion from this type of data, which tends to dramatically overcall the potential for toxicity.

There is also animal data, which is more relevant but still not definitive. Typically several animal models are used, and exposure levels are high. Animal models are useful in determining safety limits for dosing. Remember, everything is a toxin at high enough dose. The question is not, is a substance toxic, but at what dose does it become toxic. That is what animal studies are for.

They are also useful for determining what type of toxicity occurs – which organ systems are affected and by what kind of damage (cancer vs organ failure, for example).

In vitro and animal data are useful in informing safety limits for substances and to inform what kind of toxicity we should be looking for. They can also determine if it is safe to conduct human trials, when dealing with things like pharmaceuticals.

Human data, however, is the most important for definitively answering the question. Human data is mostly either directly experimental (as with drugs) or epidemiological (for chemicals that are used in industry but not intended for human use).

Regarding human epidemiological studies of glyphosate, a 2016 systematic review and meta-analysis concluded:

Meta-analysis is constrained by few studies and a crude exposure metric, while the overall body of literature is methodologically limited and findings are not strong or consistent. Thus, a causal relationship has not been established between glyphosate exposure and risk of any type of LHC.

A 2012 systematic review of glyphosate and cancer concluded:

Our review found no consistent pattern of positive associations indicating a causal relationship between total cancer (in adults or children) or any site-specific cancer and exposure to glyphosate.

They also recommended that future studies use biomonitoring to measure actual exposure levels.

Despite this the International Agency for Research on Cancer (IARC) in 2015 categorized glyphosate as a probable carcinogen, going against many other expert panel reviews. This garnered a lot of press attention, and became a standard talking point of anti-GMO activists. The IARC, however, has a reputation of calling almost anything a carcinogen.

A 2016 expert panel review conducted their own review of the evidence.

The Expert Panel concluded that glyphosate, glyphosate formulations, and AMPA do not pose a genotoxic hazard and the data do not support the IARC Monograph genotoxicity evaluation. With respect to carcinogenicity classification and mechanism, the Expert Panel concluded that evidence relating to an oxidative stress mechanism of carcinogenicity was largely unconvincing and that the data profiles were not consistent with the characteristics of genotoxic carcinogens.

In fact, four independent expert panels reviewed the IARC conclusion, and found:

The overall weight of evidence from the genetic toxicology data supports a conclusion that glyphosate (including GBFs and AMPA) does not pose a genotoxic hazard and therefore, should not be considered support for the classification of glyphosate as a genotoxic carcinogen. The assessment of the epidemiological data found that the data do not support a causal relationship between glyphosate exposure and non-Hodgkin’s lymphoma while the data were judged to be too sparse to assess a potential relationship between glyphosate exposure and multiple myeloma. As a result, following the review of the totality of the evidence, the Panels concluded that the data do not support IARC’s conclusion that glyphosate is a “probable human carcinogen” and, consistent with previous regulatory assessments, further concluded that glyphosate is unlikely to pose a carcinogenic risk to humans.

An independent 2016 expert review found:

Only the Agricultural Health (cohort) Study met our a priori quality standards and this study found no evidence of an association between glyphosate and NHL. For MM, the case control studies shared the same limitations as noted for the NHL case-control studies and, in aggregate, the data were too sparse to enable an informed causal judgment. Overall, our review did not find support in the epidemiologic literature for a causal association between glyphosate and NHL or MM.

For non-cancer outcomes the epidemiology is similar. A 2012 review of developmental outcomes concluded:

In conclusion, the available literature shows no solid evidence linking glyphosate exposure to adverse developmental or reproductive effects at environmentally realistic exposure concentrations.

A 2011 review of glyphosate and all non-cancer outcomes found:

Our review found no evidence of a consistent pattern of positive associations indicating a causal relationship between any disease and exposure to glyphosate.


I am simply searching through PubMed to find reviews of the safety of glyphosate, and this is what I find. You can do the same, it’s a user-friendly searchable database. There is a remarkable consistency to the reviews – they all agree that the evidence does not support an association between glyphosate exposure and any adverse health outcome. The IARC are the only outliers, and yet their flawed and quirky conclusion is the one that garnered the most attention.

There is also a theme in the reviews that we could use more and better quality studies. To put that into context, however, that is almost always the conclusion of such reviews. It is difficult to prove a negative – a lack of a correlation. Such a negative conclusion is only as good as the data supporting it, and therefore the more and more rigorous the data the better the conclusion.

We can always use more and better data when it comes to safety, but the existing data is robust, consistent, and independently replicated, and includes both glyphosate and formulations with glyphosate.

Glyphosate, in fact, is one of the safer pesticides in use (including many organic pesticides). It has replaced far more toxic herbicides. Opposing glyphosate because of unwarranted fears of toxicity is likely to cause harm due to whatever replaces it. Tilling is bad for the soil and releases CO2 into the atmosphere, and we cannot feed the world through hand weeding. Herbicides have to be part of the equation, and glyphosate is one of the safest out there.

Like this post? Share it!

31 responses so far