Does methadone have a opiate blocker in it?

Unlike actual opioid antagonists (such as naltrexone and naloxone), which bind to opioid receptors, preventing binding by opiates (thus, blocking opiates and their effects), methadone only partially blocks other opiates.

Methadone is an opioid agonist, meaning that it binds with and activates opioid receptors. However, during the time that it occupies the opioid receptor, it competes with other opiates, and prevents them from binding.

While, at a receptor level, this produces a partial "blocking" effect, the overall availability of methadone throughout the system would have to be significant enough to produce this effect everywhere in order to truly "block" other opiates.

So, the short answer is: partially