Last time I was tested for a job (Home Depot in Rhinelander), they used a dipstick test that came up hot for opioids (I was on Vicodin for a back injury at the time). They asked me to bring in the prescription bottle, and sent the sample out for further testing to determine if the actual amount was within therapeutic levels. I get tested occasionally by my psychiatrist because I am on benzos, and they need to check both whether I am taking too much of them, or taking too little of them, which might mean I am selling them. I came up low on the Lorazepam last time I was tested, as I don't always take as much as prescribed. My psychiatrist agreed with my reasons for sometimes taking less and actually thought it was a good thing.
I'm on Ritalin now. Have been for a month, and I already called her and told her I'm taking less than prescribed, which she was fine with once I told her it was because I don't always need the full dosage, and I sort of take it as needed...basically, if I need to really focus, or feel particularly out in the fog and can't hold a thought in my head.
She's OK with that and glad I told her because she would've been very concerned if I showed up low on a pee test because Ritalin, like my benzos, is a very desirable drug of abuse.