Persistence. Metronidazole is potentially persistent.
Bioaccumulation. Metronidazole has low potential for bioaccumulation.
Toxicity. Metronidazole has low chronic toxicity.
Risk. See the report from Goodpoint 2018.
This summary information about hazard comes from Fass.The information about risk is from the report from Goodpoint 2018, see below.
Fass environmental information for Flagyl (metronidazole) from Sanofi AB (downloaded 2022-04-20).
Persistence: "Ready degradability. Test results showed 1 % degradation in 28 days. (OECD 301)"
Bioaccumulation: "Partitioning coefficient: logP = -0.15 (pH unknown, source ALOGPS)."
Chronic toxicity: There are data for 3 trophic levels, most sensitive Cyanobacteria EC10 13500 microg/L.
PEC/PNEC is based on sales data in Sweden in year 2019. PEC/PNEC = 0.00015 which gives the risk insignificant. "To calculate the PNEC from an anti-microbial effect study with cyanobacteria, a default assessment factor of 10 is applied to the EC10. The EC10 is preferred over the NOEC for PNEC derivation, even if the former is higher than the latter."
There is a risk of selecting resistant bacteria with the concentrations measured in Swedish wastewater treatment plants. However, the estimate of selective concentration is very uncertain for metronidazole and measurements have not been taken in recent years. Metronidazole has previously been found in purified wastewater and surface water in Region Stockholm (2005–2012).
Comparative assessment of environmental risk and risk of resistance selection in human use of ivermectin, metronidazole, lymecycline and azelaic acid in Sweden (Report Goodpoint 2018). The risk is clearly highest for lymecycline based on measured concentrations of tetracycline in uncleaned waste water that exceeds experimental established selective levels for antibiotic resistance in bacteria. A similar but lower risk profile exists for metronidazole based on total use but with significantly lower empirical support for the efficacy The topical use of metronidazole results in a negligible risk of resistance selection in waste water treatment plants. The risk of effects of human use of ivermectin is also low, given an expected contribution to exposure in aquatic environments at subnanogram levels, and a majority of efficacy studies that show significantly higher levels of efficacy. Since there is (at least) a study that reports effects at 1 pg/L, however, one can not completely rule out risk. For azelaic acid there is nothing that indicates an environmental hazard.
Author: Health and Medical Care Administration, Region Stockholm