Pre-employment drug test forgot to tell them I had a valium prescription?

[sprinkles]

I took a pre-employment drug test today for a hospital job and forgot to tell them that I have a prescription for valium. I don't take it very often but I took one last thursday night. I know that valium shows up as benzos so I am very worried. Should I call them tomorrow and let them know that I have a prescription for it, or should I wait to see if I pop? They asked me what prescriptions I took and I only told them the daily script I take. They gave me a form to have my doctor fill out to state that my prescription would not hinder my work performance, should I ask him to add the valium to that letter as well? I'm so nervous now because I do not want them to take away my job offer because I forgot to tell them about this prescription. Thanks in advance,