Introduction: Due to the restrictive use of critical antibiotics by veterinarians in Europe, the use of first-line antibiotics such as the combination of sulfonamides (S) and trimethoprim (TMP) is likely to increase, especially in pig production. The ratio of TMP:S doses in nearly all marketed veterinary formulations is 1:5, which is supposed to produce an in vivo synergistic ratio of plasma concentrations of 1:19. However, sulfonamides drugs are quite different and lead to major differences in PK profiles, challenging the occurrence of this constant ratio of 1:19 for TMP:S in plasma. Objectives: This study aims to develop a modern population PK model for sulfadiazine (SDZ) and sulfadimethoxine (SDMX) in association with TMP to compare the evolution of the TMP:S ratios between these 2 sulfonamides over time. Materials and Methods: 16 pigs were used (8 pigs for SDZ/TMP and 8 pigs for SDMX/TMP). Each association was given by IV, oral (gavage) and IM route (only for SDZ) in a cross-over design at the recommended dosing (one dose per route of administration). A rich blood sampling was carried-out and plasma samples were assayed thanks to a sensitive HPLC method. A non-linear mixed effect modeling approach was used to estimate PK parameters (typical values and inter-individual variabilities) for each drug and compute the evolution of the TMP:S ratios over time via MonteCarlo simulations (Monolix Suite v2021R2). Results: A bi-compartmental model best described the PK data for each of the 3 drugs. SDZ and SDMX have significant different half-lives (median values of 4.1 h and 13.5 h, respectively) in pigs. The half-life for TMP was of the same order as for SDZ (median value of 2.3 h). Based on the population PK parameters, simulations (n = 500 pigs) were performed with the conventional dosing regimen (25:5 mg/kg of Sulfonamides:TMP over 5 days by oral route). The average ratio over 140 h of free TMP:S concentrations stayed between 1:10 and 1:100 for 92% and 82% of pigs, regarding SDZ and SDMX respectively. Concerning the duration of the TMP:S ratio being between 1:10 and 1:100 (over 140 h), the median time was 65 h (i.e. 46%) and 38 h (i.e. 27%) for SDZ and SDMX respectively, with a high inter-individual variability. Conclusion: The in vivo ratio of TMP:S varies greatly over time between sulfonamide drug and between individual, questioning the current formulations and dosing regimens, especially for SDMX/TMPFurther work with a third sulfamide (sulfamethoxazole) is ongoing.