By Kelly A. Reynolds, MSPH, PhD

What happens in sewage doesn’t always stay in sewage. A recently published study found that traces of the drug methadone excreted in wastewater supplies ultimately impact sources of drinking water. Exposure to methadone, however, is not the primary human health concern. Rather, the interaction between methadone and disinfection byproducts that produces the carcinogenic compound NDMA (N-nitrosodimethylamine) currently has health officials taking notice.

What your sewage says about you

Scientists can tell a lot about you from your feces. Reportedly, monitoring sewage for specific genetic markers was considered (if not attempted) in tracking Osama bin Laden’s specific location.(1) Given that each individual has a unique DNA sequence that is naturally shed in urine and feces, if you know something about their baseline genetic fingerprint, sewage becomes the perfect sample to secretly collect. In fact, the field of ‘sewage epidemiology’ is exploding. We can gather evidence from wastewater to track illness rates or even drug use in a community or, theoretically, at the individual household level.

While the two don’t seem connected, the presence of pharmaceuticals in wastewater does affect the quality of drinking water. Pollutants in wastewater do not stay localized. Wastewater treatment plant effluent is eventually released back into the environment but some contaminants of concern remain. In fact, wastewater facilities are a major contributor of pharmaceutical residuals in drinking water sources. The connection between our toilets and taps is increasingly obvious but the resulting human health effects from exposure to lingering contaminants are still uncertain.

Opioid narcotics in drinking water sources
V57_N10_Reynolds Pull-quote

Methadone is an opioid narcotic used to treat symptoms of heroin withdrawal and chronic pain and it is commonly found in wastewater and drinking water sources. At medically relevant doses, the drug can cause severe side effects, including breathing abnormalities, allergies, heart arrhythmia and addiction. About four million prescriptions per year are issued in the US. Like many drugs, not all of methadone is absorbed into the body during treatment. Approximately 30 percent of the chemical is rapidly excreted in urine, which ends up in the wastewater treatment plant and eventually in the general environment.

Concentrations of pharmaceuticals in drinking water are orders of magnitude less than prescription doses. Methadone, for example, is prescribed at doses ranging from 2.5 mg every 12 hours for pain to 120 mg per day for opioid detoxification. Although the risks of environmental dose are lower than treatment level doses,consumers should still be informed of potential hazards and solutions to prevent lifelong, low-level exposures.

Little is known about the direct or indirect impact of pharmaceuticals and illicit drugs in drinking water, especially when exposure occurs routinely to a wide range of low-dose hazards simultaneously. What we do know is that they are present and as physicians and drug companies continue to expand the use and availability of drug treatments, concentrations of these drugs and their metabolic byproducts continues to increase in the environment.

Evidence of wastewater contamination

One study monitored pollutants daily for a week at wastewater treatment plants in two communities near Albany, New York.(2) Six drugs, including methadone and cocaine, and eight related metabolites were targeted. Concentrations of drugs in the sewage were used to estimate consumption rates in the community. The environmental emission levels of illicit drugs ranged as high as 67.5 mg per day per 1,000 people relating to an estimated usage ranging from 1.7 to 3,510 mg per day per 1,000 people.

Wastewater treatment plants were not designed to remove these types of contaminants from water and report a removal efficiency of nearly zero for methadone, methamphetamine and others. Contaminants not removed from wastewater are eventually discharged into the environment and make their way into drinking water sources.

Another study monitored effluent from three wastewater treatment plants over five years.(3) Two of the treatment plants received waste inputs from pharmaceutical manufacturing facilities. Results from this long-term analysis were compared to baseline effluents from 23 wastewater treatment plants throughout the nation. Generally, targeted drug concentrations were very low (< 1 ug/L). However, in the two effluents from the pharmaceutical-industry impacted sites, concentrations of oxycodone and methadone were as high as 1,700 ug/L. This level is 1,000-fold higher than typical wastewater facilities.

Hazardous metabolites

Levels of methadone and other pharmaceuticals in drinking water are thought to be too low to cause human health concerns.(4) Exposure to the methadone, however, is not the primary concern in drinking water. Rather, it is the interaction between methadone and drinking water disinfectants, such as chloramines, that produces the toxic metabolite NDMA. NDMA is an organic chemical commonly found in drinking water and food (beer, milk, cured meats).

First identified in California waters impacted by rocket fuel manufacturing wastes, NDMA has been detected at levels as high as 40,000 ng/L in environmental waters. NDMA clearly causes cancer in animals and is thought to be a potent human carcinogen as well. Exposures have been linked to stomach and colon cancers. Although currently unregulated by the US EPA in drinking water, numerous studies have found NDMA levels in drinking water above recommended levels thought to result in adverse health effects. Methadone may account for up to 62 percent of the NDMA formation in wastewater, and about 25 percent of the US population consumes drinking water treated with chloramines.

A recently published study found methadone at a median concentration of 23 ng/L (ranging from 1 to 2,256 ng/L) and contributing up to 10 percent of NDMA formation potential in surface waters used for drinking.(5) The authors conclude that beneficial medicinal applications result in the potential exposure of millions of tap water consumers to harmful carcinogens, including NDMA.

Contaminant removal

Currently, there is no maximum contaminant level (MCL) for NDMA in water but California health agencies proactively set a public health goal of 3 ng/L and a response level of 200 ng/L. The response level correlates to an estimated one in 10,000 probability of causing cancer and a recommendation of removing the water source from service. Federal MCLs are generally set at values corresponding to risks no greater than one in a million.

Once formed, NDMA does not rapidly degrade in the environment and absorbs poorly to media. Thus, the contaminant passes easily through soils and activated carbon. Ultraviolet light can be used to degrade NDMA by breaking the structural bonds of the compound and sunlight can reduce NDMA in the environment. Reverse osmosis has been shown to remove NDMA from drinking water but only at an efficiency of about 50 percent.

The best approach to minimize NDMA formation is the use of activated carbon prior to disinfection. Unfortunately, only about 10 percent of treatment plants have this process in place and upgrades are costly, approaching tens of millions of dollars. While POU devices can help to reduce exposures to NDMA and other wastewater metabolites, a larger effort is needed to prevent their formation in wastewater and to minimize their transport to drinking water sources.


  1. Ferguson, B. Spies in the Sewer, [Online]. Available: [Accessed 13-9-2015].
  2. Subedi, B. and Kannan, K. “Mass Loading and Removal of Select Illicit Drugs in Two Wastewater Treatment Plants in New York State and Estimation of Illicit Drug Usage in Communities through Wastewater Analysis,” Environmental Science and Technology, vol. 48, no. 12, pp. 6661-6670, 2014.
  3. Phillips, P.J.; Smith, S.G.; Kolpin, D.W. et al. “Pharmaceutical Formulation Facilities as Sources of Opioids and Other Pharmaceuticals to Wastewater Treatment Plant Effluents,” Environmental Science and Technology, vol. 44, pp. 4910-4916, 2010.
  4. Chawaga, P. Methadone In The Water: What’s The Real Risk? 8-8-2015. [Online]. Available: [Accessed 13-9-2015].
  5. Hanigan, D.; Thurman, E.M.; Ferrer, I. et al. “Methadone Contributes to N-Nitrosodimethylamine Formation in Surface Waters and Wastewaters during Chloramination,” Environmental Science and Technology Letters, vol. 2, no. 6, pp. 151-157, 2015.

About the author
Reynolds_Kelly_mugDr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at


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