**Background:** Pharmaceuticals are recognized as emerging contaminants in aquatic environments, with amoxicillin (AMX) and carbamazepine (CBZ) among the most frequently detected. Despite widespread occurrence, data on chronic and mixture toxicity to marine organisms remain limited. This study aimed to assess the acute and chronic effects of AMX and CBZ, individually and in a 1:1 mixture, on the marine copepod Tigriopus fulvus, a species suitable for laboratory bioassays and ecologically important as a trophic link.
**Methods:** Acute toxicity tests (96 h) were conducted on nauplii (≤24 h old) exposed to nominal concentrations of 6.25, 12.5, 25.0, 50.0, and 100 mg/L of AMX, CBZ, and their 1:1 mixture. Chronic toxicity tests (28 days) used a full life-cycle approach with nominal concentrations of 0.1, 1, 10, and 100 μg/L for single compounds and the mixture. Eight endpoints were examined: lethality, percentage of nauplii reaching copepodite stage after 5 days, development time to maturation, sex ratio, hatching time, mean broods per female, mean nauplii per brood and per female, and aborted egg sacs. Chemical analysis via HPLC-DAD confirmed measured concentrations. Statistical analyses included ANOVA with Tukey's post hoc test and calculation of NOEC, LOEC, and MATC values.
**Key Results:** Acute exposure: No LC50 values could be determined as mortality did not reach 50% at any tested concentration. The highest mortality was 44% at 100 μg/L CBZ and 22% at the highest mixture concentration. AMX showed no effect even at 100 mg/L. Chronic exposure: Survival remained >95% across all treatments. Larval development (copepodite stage at day 5) was significantly inhibited at the highest AMX concentrations (9.540 and 93.40 μg/L) and CBZ concentrations (8.88 and 95.6 μg/L), with 41% and 37% development, respectively, versus 63±4% in controls. Stimulatory effects were observed at the lowest concentrations. Hatching time increased significantly from 2.4±0.2 days in controls to 3.2±0.4 days at 100 μg/L AMX, with MATC values of 0.25 μg/L (AMX), 3.34 μg/L (CBZ), and 0.27+0.29 μg/L (mixture). Mean broods per female decreased from 5.6±0.5 in controls to 3.9±0.7 at 93.40 μg/L AMX; MATC values were 0.25 μg/L (AMX), 0.11 μg/L (CBZ), and 0.27+0.29 μg/L (mixture). Total nauplii per female decreased significantly at 9.540 and 93.40 μg/L AMX (88.1 and 73.9 vs. 113.5±24.6 in controls) and in the mixture from 0.94+1.03 μg/L upward. Aborted egg sacs increased significantly at the highest concentrations of AMX and CBZ and in the mixture from 9.2+8.7 μg/L upward. MATC values for aborted sacs were 29.8 μg/L (AMX), 29.1 μg/L (CBZ), and 2.94+3.00 μg/L (mixture). No significant effects were observed on sex ratio or development time to maturation.
**Clinical Implications:** Although AMX and CBZ are not acutely toxic to T. fulvus at environmentally unrealistic concentrations, chronic exposure at low μg/L levels—within or near environmentally relevant ranges—can impair reproduction and development. The mixture often showed effects at lower concentrations than single compounds, highlighting the importance of considering combined exposures in risk assessment. Declining fertility and delayed development could have population-level consequences for copepods and, by extension, for higher trophic levels that depend on them as prey. These findings support the inclusion of sub-lethal, multi-endpoint chronic tests in environmental risk assessment frameworks for pharmaceuticals, particularly for marine ecosystems.