The T-value in the score for hazard refers to chronic toxicity. Underlying data for P, B and T are from Fass.
Measurements in Swedish purified wastewater show concentrations between 13 and 62 ng/L. Mirtazapine is not microbiologically readily degradable, but is broken down by sunlight. Studies of expected mechanisms of action in aquatic organisms are lacking.
In comparison with citalopram, fluoxetine, sertraline and venlafaxine, the effects of mirtazapine on aquatic organisms are studied to a lesser extent and not by equally relevant methods.
Environmental impacts of several SSRIs / SNRIs exceed the reported levels which affect aquatic organisms, especially invertebrates. However, the studies reporting the lowest effect concentrations are questioned, which makes the risk assessment uncertain. These drugs have also been found in wild fish and/or in fish exposed to purified wastewater including sertraline and citalopram at concentrations close to therapeutic concentrations in humans. The risk of mirtazapine accumulating in fish at potent levels is estimated to be lower than for the comparable substances.
Even bioaccumulation data and data on the potential of the substances suggest that mirtazapine in the environment presents a significantly lower risk than sertraline and citalopram and a lower risk than fluoxetine and venlafaxine.
Fass environmental information for Remeron-S (mirtazapine) (downloaded 2019-04-25).
Persistence: "Photolysis: Half-life = 0.35 hours at pH 7 (US FDA 3.10) (Ref. X) Justification of chosen degradation phrase: Mirtazapine does not pass the ready biodegradation test but is inherently degradable in acclimated biological systems. The DT50 ≤ 120 d for the total system, therefore the phrase “Mirtazapine is slowly degraded in the environment” was chosen."
Bioaccumulation: "Steady State BCF = 201 to 239 (OECD 305). Since BCF < 500, the substance has low potential for bioaccumulation."
Toxicity: There are data for 3 trophic levels, most sensitive crustacean (Daphnia magna) "NOEC (21 day) (reproduction and growth) = 320 μg/L (OECD 211)".
PEC/PNEC is based on sales data in Sweden in year 2015. PEC/PNEC = 0.005 which gives the risk insignificant, i.e. consideration has not been given to measured levels in the environment.
Author: Health and Medical Care Administration, Region Stockholm