LINCOLN, Neb. (CN) — Despite Monsanto's claim that its Roundup weed-killer is "safe enough to drink," four Nebraska farmers say the widely used herbicide gave them non-Hodgkin's lymphoma.

The World Health Organization's International Agency for Research on Cancer classifies glyphosate as "probably carcinogenic to humans" but Monsanto has promoted Roundup as "safe enough to drink," the farmers say in their federal lawsuit.

"Roundup is used by Nebraskans raising everything from grain to grass and tulips to trees. Nothing on the label alerts users to health risks," their attorney David Domina said in an interview. "Nebraskans deserve the benefit of the WHO research, and protection against unknown exposure."

Monsanto, based in St. Louis, is the world's largest producer of glyphosate herbicides. The company made $4.8 billion from Roundup sales last year. It is applied to "Roundup-ready" crops that are genetically modified to resist it.

Ubiquitous in hardware stores for home use nationwide, Roundup is used in commercial agriculture on more than 100 varieties of crops. More than 85 million lbs. of glyphosate was applied to U.S. crops in 2001, and its use more than doubled, to 185 million lbs., by 2007, according to the complaint.

"Glyphosate is found in rivers, streams, and groundwater in agricultural areas where Roundup is used. It has been found in food, the urine of exposed persons, and in the urine of urban dwellers without direct contact with glyphosate," the 48-page complaint states.

California last year became the first state to label Roundup as a carcinogen, based on the IARC research, and Monsanto then sued the state to fight the designation.

All four of the plaintiffs — Larry Domina, Frank Pollard, Robert Dickey, and Royce Janzen — say they were diagnosed with non-Hodgkin's lymphoma after being exposed to Roundup year after year. They claim, among other things, that Monsanto mislabeled the product, in defiance of the "body of recognized scientific evidence linking the disease to exposure to Roundup."

According to the IARC, cancers most associated with glyphosate exposure are non-Hodgkin's lymphoma and other blood cancers, including lymphocytic lymphoma / chronic lymphocytic leukemia, B-cell lymphoma, and multiple myeloma.

The farmers seek punitive damages for defective design, failure to warn, negligence and breach of warranty.Attorney Domina, of Omaha, is assisted by Robin Greenwald with Weitz & Luxemborg in New York City.