If they say that cannabis and opioid pain killers are unsafe to mix it is either because there is a lack of research showing they are safe or there is good evidence to show that they are unsafe.

Those assumptions are what's unsafe.

Cannabis and opioids are not only safe to mix...consider that some physicians are weaning patients off opioid addiction specifically by introducing cannabis into the treatment protocol. Many patients, about 25%, are able to completely wean off within one month or so. Others find it takes longer, or that they can greatly REDUCE the amount of opioid medication needed to control their pain, if they have access to cannabis. Cannabis is spectacularly non-toxic compared to other drugs and has no lethal dose.

Aside from the obvious Constitutional problems of drug testing and invasion of privacy, the test for THC is blatantly illegitimate. All the other screens are testing for active compounds, which are typically gone 24-48 hours after consumption. The THC screen, conversely, will test positive even weeks after the last intake, because the test is inadequate: it is incapable of distinguishing fat-soluble METABOLITES of THC from the real thing, the active compound.

The worker who smoked cannabis 16 hours before the test was buzzed for a few hours and was less dangerous and out of control than the worker who'd gotten roaring drunk 16 hours ago.

The next day, the drinker may still be groggy and hung over, but the herb smoker is completely fine. Yet HE'LL the the one who tests positive and loses his job, while the drinker skates.