How much of the least harmful substance do you need to give someone to fail a drugs test?

Like some poppy seeds on some bread? Actually harmless levels.

Opiates are out of your system relatively quickly. You’d have to be tested within a couple days of consumption to detect it. THC and it’s metabolites, however, bind with fats in your body and stick around for much longer, especially so for regular and chronic users. A single dose can be reliably detected more than a week after consumption, with chronic use detectable for months after the last dose.

So, you’re saying you only need a little dose in order to fail the drug test?

So some very very light brownies would do, or?

That would be a win win. Theoretically.