The production of coca and poppy as well as the processing and production of cocaine and heroin involve significant environmental impacts. Both coca and poppy are grown intensively in a process that involves the clearing of land in remote areas, the planting of the crop, and protection against pests such as weeds, insects, and pathogens. The aerial spray program to control coca and poppy production in Colombia with the herbicide glyphosate is conducted with modern state-of-the-art aircraft and spray equipment. As a result of the use of best available spray and navigation technology, the likelihood of accidental off-target spraying is small and is estimated to be less than 1% of the total area sprayed. Estimated exposures in humans resulting from direct overspray, contact with treated foliage after reentry to fields, inhalation, diet, and drinking water were small and infrequent. Analyses of surface waters in five watersheds showed that, on most occasions, glyphosate was not present at measurable concentrations; only two samples had residues just above the method detection limit of 25 microg/L. Concentrations of glyphosate in air were predicted to be very small because of negligible volatility. Glyphosate in soils that are directly sprayed will be tightly bound and biologically unavailable and have no residual activity. Concentrations of glyphosate plus Cosmo-Flux will be relatively large in shallow surface waters that are directly oversprayed (maximum instantaneous concentration of 1,229microgAE/L in water 30cm deep); however, no information was available on the number of fields in close proximity to surface waters, and thus it was not possible to estimate the likelihood of such contamination. The formulation used in Colombia, a mixture of glyphosate and Cosmo-Flux, has low toxicity to mammals by all routes of exposure, although some temporary eye irritation may occur. Published epidemiological studies have not suggested a strong or consistent linkage between glyphosate use and specific human health outcomes. An epidemiology study conducted in Colombia did not show any association between time to pregnancy in humans and the use of glyphosate in eradication spraying. The mixture of glyphosate and Cosmo-Flux was not toxic to honeybees. The mixture was, however, more toxic to the alga Selenastrum, the cladoceran Daphnia magna, fathead minnow, and rainbow trout than formulated glyphosate (Roundup) alone. Studies on the use of glyphosate in agriculture and forestry have shown that direct effects on nontarget organisms other than plants are unlikely. Indirect effects on terrestrial arthropods and other wildlife may be the result of habitat alteration and environmental change brought about by the removal of plants by glyphosate. Because of the lack of residual activity, recovery of glyphosate-treated areas in Colombia is expected to be rapid because of good plant growth conditions. However, return to the conditions of tropical old-growth forest that existed before clear-cutting and burning may take hundreds of years, not from the use of glyphosate but because of the clear-cutting and burning, which are the primary cause of effects in the environment. The risk assessment concluded that glyphosate and Cosmo-Flux did not present a significant risk to human health. In the entire cycle of coca and poppy production and eradication, human health risks associated with physical injury during clear-cutting and burning and the use of pesticides for protection of the illicit crops were judged to be considerably more important than those from exposure to glyphosate. For the environment, direct risks from the use of glyphosate and Cosmo-Flux to terrestrial mammals and birds were judged to be negligible. Moderate risks could occur in aquatic organisms in shallow surface waters that are oversprayed during the eradication program. However, the frequency of occurrence and extent to which this happens are unknown as data on the proximity of surface waters to coca fields were not available. Considering the effects of the entire cycle of coca and poppy production and eradication, clear-cutting and burning and displacement of the natural flora and fauna were identified as the greatest environmental risks and are considerably more important than those from the use of glyphosate for the control of coca and poppy.