Some painkillers have cross reactivity. please explain this to me. how does this effect meth drug test? is it when painkillers are taken together?

Drug reactivity?: Seems like you may be asking 2 questions, so lemme try to answer both. Narcotics & opioids interact w/ several receptor subtypes, the most common being delta, kappa, mu & OR1, with differing affinities/binding strengths like lock & key. These pain receptors are located in different parts of your body in different amounts. UDS or blood tests test for these drugs and also separately for amphetamines