Basic Opioid Terminology:
Opioid: Any chemical- natural, semi-synthetic or synthetic that acts on the endogenous opioid system.
Opiate: Any naturally occurring opioid. Morphine, Codeine are Opiates.
Semi-Synthetic: Opioids that are closely related to Opiates, generally only a couple steps removed. Examples include Diacetylmorphine, Nicomorphine, Dihydrocodeine, Ethylcodeine, etc.
Synthetic: Opioids that are not derived from natural Opiates or semi-synthetic Opioids. Open-chain class like Methadone, Dextromoramide, the Fentanyls like Sufentanil, Meperidine and the Pethidine class like MPPP, etc.
Partial Agonists: Opioids, generally if not always synthetics, like Buprenorphine, Butorphanol, Pentazocine, etc that are agonists at one or more opioid receptors while antagonists at other receptors.
Methadone is a full mu-agonist synthetic opioid. It has no antagonist properties.
The "Blocking" noticed with Methadone during Methadone Maintanence Therapy (MMT) with higher doses (over 60mg/day usually) is due to tolerance- not a specific action on the part of Methadone itself.
A person taking any mu-agonist opioid at moderately high to high doses every day will notice a similar effect. High doses of Morphine, Hydromorphone, Meperidine, etc will "block" the effects of another opioid- if the dose is high enough.
Do not confuse this with the action of partial agonists and antagonists.
Buprenorphine 'blocks' other opioids due to its ability to outcompete for the mu opioid receptors. The bind Buprenorphine exhibits on mu-receptors is stronger than that of most mu-agonist (Fentanyl, Heroin, Morphine, Methadone, Meperidine) and antagonist (Naloxone, Naltrexone, Nalorphine) drugs. This is why different methods are necessary to treat a Buprenorphine overdose than a Heroin overdose (The latter can be treated with one or two injections of Naloxone, the former requires an IV infusion of Naloxone or use of a stronger and longer acting antagonist like Nalmefene, or non antagonist therapies).
Mixing Methadone and any other mu-agonist opioid for the purpose of going above your Methadone induced opioid tolerance to get high is very, very dangerous and foolish.
Because you don't know exactly how much Fentanyl (or Morphine or Heroin or Hydromorphone, etc) it will require to go above your Methadone tolerance, you are gambling with your health. The difference between too little and too much of the second opioid can be a very small dosage.
Plus, the respiratory depression and other side effects will not be lowered due to your tolerance. The difference between enough Fentanyl to get high and enough to cause loss of consciousness and death from respiratory depression is likely very small.
![:\](https://bluelight.org/xf/BL_Images/Orig_Emoji/wrygrin.gif "wry grin :\")
