In 2015 the World Health Organization’s International Agency for Research on Cancer issued a report identifying glyphosate, malathion and diazinon as probable carcinogens and tetrachlorvinphos and parathion as possible carcinogens (Roundup contains glyphosate but also inert ingredients, some of which the manufacturer is allowed to keep secret):
A Working Group of 17 experts from 11 countries met at the International Agency for Research on Cancer (IARC) on 3–10 March 2015 to review the available published scientific evidence and evaluate the carcinogenicity of five organophosphate insecticides and herbicides: diazinon, glyphosate, malathion, parathion, and tetrachlorvinphos. A summary of the evaluations has now been published in The Lancet Oncology. The detailed assessments will be published as Volume 112 of the IARC Monographs….
Download the IARC report here.
This photo of Roundup-induced skin damage from Wikimedia Commons is not necessarily related to cancer but is certainly a warning sign: