by Brenda Salgado

Aromatase inhibitors (AIs) are approved treatments for estrogen-receptor-positive breast cancer. Aromatase is an enzyme that helps to convert testosterone into estrogen, and AIs lower risk of recurrence by inhibiting aromatase activity.

Given this, would it surprise you to learn the following?

  • Atrazine, a widely used weed killer, is an aromatase promoter.
  • In 2000, Novartis and AstraZeneca formed Syngenta through the merger of their agricultural divisions.
  • Syngenta sells atrazine, and Novartis and AstraZeneca sell aromatase inhibitors.
  • The United States uses about 80 million pounds of atrazine every year. Atrazine was denied regulatory approval in the European Union.


I interviewed Tyrone Hayes, a professor of integrative biology at the University of California, Berkeley, about research on atrazine’s health effects and his experience with corporate influence at universities.

BS: Why do you study atrazine?

TH: My interests include hormone-disrupting chemicals and effects on amphibian development. Years ago we developed an assay to screen environmental estrogens. Then Novartis contracted with me to study if atrazine is an endocrine disruptor in amphibians.

BS: What effects have you and others found?

TH: In my lab, atrazine caused male frogs to grow ovaries and eggs. This was at very low levels—0.1 parts per billion. It feminized them by increasing estrogen.

One of the first researchers studying this found that alligator embryos exposed to atrazine had altered sexes and abnormal sex organs. Numerous animal studies have documented pregnancy loss, damage to nerves, immune suppression, and various cancers. Effects have been seen in a wide variety of species: fish, amphibians, reptiles, and rodents. More important, animal studies show exposure in the womb can contribute to disease later in life.

BS: What about humans?

Effects in the lab are consistent with associations seen in humans. One study found that women who drank well water contaminated with atrazine were more likely to develop breast cancer than women who lived in the same area but were unexposed. Another study of Syngenta factory workers found that employees exposed to atrazine had an 8.4-fold increase in prostate cancer compared to unexposed workers. Other research found atrazine is associated with impaired fertility and low sperm counts. Studies have also found that atrazine promotes aromatase in human cancer cell lines.

BS: Are effects documented at levels seen in the environment?

Effects are seen at low levels (parts per billion), even in human cell lines and tissues. Atrazine metabolites have been detected in farmers’ urine, at levels up to 2,400 parts per billion. So these are physiologically relevant levels.

BS: How is atrazine used?

TH: It’s used primarily on corn, as well as soy, sugar cane, Christmas trees, and in forestry. About 80 million pounds a year are used in the U.S., with highest rates in corn-growing states like Nebraska, Iowa, Kansas, Illinois, Indiana, and Ohio. Atrazine is the most common pesticide found in ground and surface water, and has been found in rain water and snow.

BS: If aromatase inhibition is good for breast cancer treatment, how can an aromatase promoter be allowed in our food, water, and environment?

TH: That’s one thing that interests me. Syngenta produces this aromatase-promoting chemical, atrazine, and they try to downplay its negative effects in the environment and our health. [At the same time] Novartis and AstraZeneca are selling drugs to treat breast cancer that do the exact opposite. If women in the Midwest are taking an AI for breast cancer, it’s very possible they are exposed to atrazine in their water.

BS: What can you tell me about the manufacturers of atrazine and AIs?

TH: Atrazine was first produced by Geigy/Ciba-Geigy, and then they combined with Novartis, which sold both agricultural chemicals and pharmaceutical drugs at the time. AstraZeneca did too. In 2000, Novartis and AstraZeneca formed Syngenta through the merger of their agricultural divisions.

Of the three aromatase inhibitors, one is anastrozole (Arimidex), sold by AstraZeneca. Another is letrozole (Femara), which is sold by Novartis.

BS: What do you think about this connection?

TH: On the one hand, how can one company sell an aromatase inducer and argue it has nothing to do with public health, while a related company sells a compound that does exactly the opposite and say it’s great for breast cancer? Even if it’s not an intentional conspiracy, at the very least they have to take responsibility. They have to realize what they’re doing.

BS: What have been the repercussions of your work?

TH: They offered me funding to do the research, but I couldn’t publish my results without their permission because my contract had confidentiality restraints. My initial findings were noteworthy because we saw effects at low levels, but they did not want me to publish and would not approve or fund follow-up studies to confirm my results. I had to find additional money and reproduce the research. They tried to discredit my work, and my job was endangered, but I have supporters here at the university. I believe that if I did not have tenure at the time, there is a chance I might not be here now.

BS: Other scientists have had similar encounters, highlighting concerns about the integrity of science.

TH: It is changing. I can remember a time when getting tenure was mostly about how many studies you published. What I see now, they are also looking at the cash you bring to the university. It does take money to attract good scientists and teachers, to provide students with quality education, and to maintain labs. I’m not denying that. I think the troubling thing is the volumes of money coming from corporations and what they want for their investment. It’s up to the administration to protect the intellectual freedom of its faculty and students. We should be cautious. It’s not necessarily a bad thing, but we need to put protections in place.

BS: What are you working on now?

TH: We are looking at how atrazine affects immune function and cancer. Response to atrazine is affected by other stressors, including other contaminants, so we are looking at effects of exposure to several pesticides together, at levels that are seen in wildlife.

BS: What can people do?

There is a federal bill to ban atrazine, which we should support. We should also push the EPA endocrine disruptor program to complete the work they were asked to do 12 years ago.


More Online: For more information on atrazine and for citations for studies, For information on the EPA Endocrine Disruptor Screening Program, see and