 Equipotent, injected doses had comparable action courses, with no difference in subjects' self-rated feelings of euphoria, ambition, nervousness, relaxation, drowsiness, or sleepiness.Short-term addiction studies by the same researchers demonstrated that tolerance developed at a similar rate to both heroin and morphine.When compared to the opioids hydromorphone, fentanyl, oxycodone, and pethidine/meperidine, former addicts showed a strong preference for heroin and morphine, suggesting that heroin and morphine are particularly susceptible to abuse and addiction.