An accidental heroin overdose occurs if someone takes too much of the semi-synthetic opioid and experiences serious adverse effects as a result. This can happen for many reasons, including: Using heroin that is unexpectedly strong; Taking heroin that is adulterated with other substances like fentanyl; Mixing heroin with alcohol, prescriptions medications, or sedatives, all of which can increase the risk of heroin overdose; Reduced tolerance to heroin after a period of abstinence