Euphoria with opioids can always be achieved by increasing dosage because of the very process of agonizing the mu receptors and
its effect in the limbic system of the brain. After morphine and
diacetylmorphine (heroin) stopped doing it for me as effectively, I switched to fentanyl, and that hit the spot. There is always a stronger opioid (carfentanyl, an analogue of fentanyl, for example, used to tranquilize big African game, like elephants and rhinos).