 [ Symptoms ] what do i do if i missed one of my bactrim ds dosage  i was supose to take it 5 hours ago its a twice a day dosage it is a 10 day treatment and i have three more days left what do i do if i missed one bactrim ds dosage [ Disease ] i am not sure of the reason you are taking bactrim but it is okay if you have missed just the one dose this sulfa antibitoic has a long half life so it is still in your system doing its job go back on your regular dosage schedule when the time arrives