It shouldn't 'kill' the opiate high. Though if you are noticeably stimulated from the coffee, it will reduce the sedation from the opiate high. But in my vast experience, the buzz from opiates will more or less overpower a low dose of caffeine. If you are that concerned, just wait a few hours until the caffeine wears off, and you'll be good to go.. But either way you won't be wasting it.