I was prescribed 5 mg valium twice a day for 4 days. it showed up in a drug screen almost three weeks later. i'm 6'4 270lbs 27 yo male is this possib?

Absolutely: The half-life of diazepam (Valium) is very long, 20-100 hours (36-200 hours for its active metabolites). Depending on your liver function this could be easily detectable 3-4 weeks out. If your are in trouble legally or at work for this positive result, contact the doctor who prescribed the Valium to you and ask for a note explaining the reason for the Rx and outlining the long half life of the med

Drug screens: Urine drug screens have differing levels of sensitivity for each class of drug. And the manner in which drugs are metabolized in your system can vary between individuals and drugs. The urine concentrates many metabolites, and if you are a slower metabolizer, then yes, it is possible for drug use several weeks ago to still be detectable.