In one word: structure
Even intravenous methadone takes quite some time reach peak plasma, subjectively, about 70 minutes. The wonderful thing about this, is the amazing duration which it stays at peak plasma. Having abused virtually all clases of opioids, even rare esoteric opioids; I still rank IV methadone ampules/vials among the most favorable opioid commonly available. However, due to my propensity for opioid addiction I mujst abstain from even oral methadone, which i also enjoy. Morphine and DAM has its place, but the k-agonist properties can make the experience emotionally volitile. The last time I used IV street levek tar heroin (roughly six years ago), I wound uo crying like an infant. Methadone, in high doses, preferable. Levorphanol is also a drug that offers similarly enjoyable effects, with the added benefits of a" rush' of sorts. I have barred myself from using opioids of any potency, P.O. or I.V.
Methadone is worth the wait in regard to pharmacokinetics (of which there is huge amount of data online). If you are having issues with withdrawal, i suggest doing what I used to do when on MMT: set your alarm for 60-90 minutes before intended wake up time, take the tablets or liquid, sleep for another 60-90 miniutes, and upon waking you will" comfortable".