Methadone (Trade Name Physeptone in the UK) Methadone  is a synthetic opioid analgesic (pain killer) that is particularly cross-tolerant with Heroin (it fully binds to the same opiate receptors in the brain) and as such is commonly used as a Heroin substitute. It most commonly comes in the form of Methadone Linctus (a green liquid taken orally) but is also available in tablet form and injectable ampoules, though it is not commonly prescribed in this form.