The ratio of concentration of drug in co-trimoxazole trimethoprim and sulfamethoxazole is:

Trimethoprim is usually given orally, alone or in combination with sulfamethoxazole, which has a similar half-life. Trimethoprim-sulfamethoxazole can also be given intravenously. Trimethoprim is well absorbed from the gut and distributed widely in body fluids and tissues, including cerebrospinal fluid. Because trimethoprim is more lipid-soluble than sulfamethoxazole, it has a larger volume of distribution than the latter drug. Therefore, when 1 pa of trimethoprim is given with 5 pas of sulfamethoxazole (the ratio in the formulation), the peak plasma concentrations are in the ratio of 1:20, which is optimal for the combined effects of these drugs in vitro. Ref: Deck D.H., Winston L.G. (2012). Chapter 46. Sulfonamides, Trimethoprim, & Quinolones. In B.G. Katzung, S.B. Masters, A.J. Trevor (Eds), Basic & Clinical Pharmacology, 12e.