I have been on antibiotic for 1 1/2 days for severe uti. how long does it take for antibiotics to work?

Depends: It is important to finish the antibiotic once it is started.

1 - 3 days: Usually start to have effect by 1 1/2 days. Did your dr. Get a urine culture which would show whether you are on correct antibiotic. Urine culture result usually available after 24 hours, but sensitivities, indicating to which antibiotics the UTI bacteria are sensitive to takes 48 hours. May need to change antibiotics if ineffective or bacteria are resistant to your antibiotic.

Within a day: Great question. Since uti's are painful, effective therapy can alleviate this symptom within a day. However, other symptoms, depending on the severity of infection, may take up to a week (fever, malaise) or so to abate (eg.Flank pain).If symptoms smolder, inform your prescriber immedately- antibiotics may need to be adjusted. Hopefully, a urine culture was intially done to guide antibiotic choice.

Burning pain <1day: Since uti's are painful, effective therapy can alleviate the symptom of pain/burning on urination within a day. However, other symptoms, depending on the severity of infection, may take up to a week (fever, malaise) or so to abate (eg.Flank pain).If symptoms are gone in a few days, it is important to complete the prescribed course! or else you risk getting a recurrence of the infection.