Picture a red jeep bumping up and down a longish, straight, red dirt road in rural Colombia and, coming from the opposite direction, one lone motorcyclist. The two vehicles draw near and stop. “Amigo, we have problems,” says the motorcyclist to the jeep driver, “they’re fumigating.” “Where?” He gestures back in the direction the jeep has come from. “Two planes.”

That was all that was said. Nothing else required. The motorcyclist sped off to do whatever you do when you’re faced with such a terrifying prospect: planes spraying your home, land and loved ones - or the home, land and loved ones of someone you know - with a cocktail of chemicals including glyphosate.

That exchange took place somewhere east of a town named San Jose del Guaviare in Colombia’s Guaviare department, one of the biggest coca-producing regions in the country that has “likely” just overtaken Peru to reclaim “its position as the world’s principal cocaine producer”, according to InSight Crime’s interpretation of recently-released US White House statistics. The planes were taking off from San Jose and piloted by US citizens because the Colombians weren’t considered skilled enough to spray accurately enough - or so I was told.

Aerial fumigations using glyphosate - developed and patented by US-based firm Monsanto - have been carried out in Colombia for more than 20 years, and have become the cornerstone of the US-financed and -supported so-called “War on Drugs.” The stated aim is to destroy the cultivation of coca, the plant used to make cocaine, and it is estimated that at least 1.6 million hectares have been sprayed.

Many people, both in Colombia and abroad, have condemned and protested the fumigations for years. The stated reasons - aside from the fact they haven’t succeeded in eradicating coca cultivation - are legion. One such reason is that they have killed 1,000s of hectares of legal crops belonging to 1,000s of campesinos, Afro-Colombians and indigenous people, and because of devastating environmental impacts including destroying soil fertility, contaminating water, and pushing coca cultivation deeper into particularly environmentally sensitive, biodiversity-rich regions like the Amazon.

Other reasons include intensifying Colombia’s civil war, facilitating killings and abuses by paramilitaries, encouraging support for guerrillas, forcing people to flee to neighbouring Ecuador, increasing poverty, and causing appalling health impacts. Headaches, vomiting, eye irritations, skin rashes and burnings, poisoning, lower sperm counts, miscarriages, hair loss, respiratory problems including lung cancer, foetal deformations, destruction of red blood cells and mental health disorders have all been reported.

“We find significant effects of spraying campaigns on the probability of occurrence of dermatological problems (skin irritations, highlight burnings, etc.) and abortions,” found one recent, particularly controversial study co-written by Daniel Mejia, the president of the Colombian government’s Advisory Commission on Narcotics Policy, based on fumigations between 2003 and 2007. “Our results corroborate some of the results in the medical literature (e.g., the negative effects of exposure to glyphosate on dermatological problems and abortions).”

But will the fumigations now be suspended or, even better, stopped altogether? In March the World Health Organisation’s International Agency for Research on Cancer (IARC) announced the results of nearly a year’s research, published in The Lancet, which found that glyphosate is “probably carcinogenic to humans”, that it causes “DNA and chromosomal damage in human cells”, and “there is convincing evidence that [it] also can cause cancer in laboratory animals.” Colombia’s Health Ministry responded the following month by recommending the “immediate suspension” of glyphosate fumigations, and the president himself, Juan Manuel Santos, appears to agree. El Colombiano reported comments by the Conservative party president that Santos would heed the ministry’s advice, and that now appears to have been confirmed by the president himself during a speech he gave on Saturday, 9 May, at a hospital in Bogota.

“I’m going to ask the government functionaries and ministers who sit on the National Narcotics Council that in their next meeting [scheduled for 15 May] they suspend the use of glyphosate in the spraying of illegal crops,” said the president, citing the Health Ministry’s recommendation and a 2014 order by the Constitutional Court to investigate the potential risks of glyphosate fumigations as contributing to his decision.

Santos followed that statement with an appearance on Colombian television yesterday, 11 May, where he said “We must find strategies [to combat cultivation] that are more effective and cause less harm to the environment and public health.”

Monsanto has responded to the WHO’s research by calling it “Junk Science”, “biased” and “irresponsible.”

“We are outraged with this assessment,” Dr. Robb Fraley, Monsanto’s Chief Technology Officer, is quoted as saying. “This conclusion is inconsistent with the decades of ongoing comprehensive safety reviews by the leading regulatory authorities around the world that have concluded that all labeled uses of glyphosate are safe for human health. This result was reached by selective ‘cherry picking’ of data and is a clear example of agenda-driven bias.”

Others disagree. Dario Aranda, a journalist from Argentina, where glyphosate is used across almost 30 million hectares, calls the WHO’s conclusions “late” but “welcome.”

“[It] has confirmed what fumigated peoples, their neighbours fighting with them, social organisations and academics outside the business sector have been saying for more than 10 years,” he wrote in an article published by lavaca.org two days after the WHO’s research was made public. “The world’s most used herbicide affects people’s health.”

• This article was amended on 13 May 2015 to remove an erroneous reference to Monsanto in the headline.