I wanted to know how long does keflex stay in your system i want to do a urine culture on saturday but i don't want it to come back contaminated like the first one because i was taking keflex i stopped taking it for a day?

Antibiotics can make a urine culture negative (by suppressing or curing bacteria susceptible to them). However, they do not cause contaminated cultures: even if a contaminated culture occurred while taking cephalexin, that wasn't the cause. You do not need to stop taking the drug for your next urine culture. Contamination results only from not collecting a cleanly voided specimen.

Taking Keflex, if effective, will give you a negative urine culture result, not a contaminated result. Urine culture result reported as contaminated means you did not take a good sample (not cleaning your meatus well and not taking a half stream sample) and the urine had been contaminated with the genital organisms. Keflex does not stay in your body past 8 hours.