Concomitant administration with a high dose of probenecid (1 or 2 g/day) may increase serum clearance of ceftriaxone by approximately 30% and decrease elimination t½ by about 20%. In vitro studies indicated that the antibacterial activity of ceftriaxone and aminoglycosides (amikacin, gentamicin, tobramycin) may be additive or synergistic against some strains of Enterobacteriaceae and some strains of Pseudomonas aeruginosa.