Reported off‐target effects of antihistamines in humans draw interest in ecotoxicity testing of first‐ and second‐generation antihistamines, the latter of which have fewer reported side effects in humans. As antihistamines are ionizable compounds, the pH influences uptake and toxicity and thus is highly relevant when conducting toxicity experiments. Zebrafish embryo toxicity tests were performed with the 3 first‐generation antihistamines ketotifen, doxylamine and dimethindene and the 2 second‐generation antihistamines cetirizine and levocabastine at pH 5.5, 7.0 and 8.0.

We detected effects on survival, phenotype, swimming activity and heart rate for 4 antihistamines with exception of levocabastine, which did not show any lethal or sublethal effects. When compared to lethal concentrations, effect concentrations neither of phenotype malformation, nor of swimming activity or heart rate, deviated by more than a factor of 10 from lethal concentrations, indicating that all sublethal effects were fairly non‐specific. First‐generation antihistamines are weak bases and showed decreasing external effect concentrations with increasing neutral fraction, accompanied by increased uptake in the fish embryo. As a result, internal effect concentrations were independent from external pH. The pH‐dependent toxicity originates from speciation‐dependent uptake, with neutral species taken up in higher amounts than the corresponding ionic species. Cetirizine, which shifts from zwitterionic to anionic state in the measured pH‐range, did not show any pH‐dependent uptake or toxicity.