For some reason, the PDR and other sources have implied that Tramadol has antagonistic properties in the past. This is absolutely false as we all know. Because it only bonds weakly to certain opiate receptors in the brain, it's
possible that Tramadol will not be strong enough to ward off withdrawal in addicted people who have no other opiates. It will not, however, precipitate
withdrawal.
Actually, I just looked up the prescribing data, and there is no mention of antagonistic effects. Just mentions binding of the parent and M1 metabolite to opiod receptors and weak inhibition of the reuptake of norephinephrine and serotonin.