Taking just 1 large dose of a prescription opioid could cause serious breathing problems that lead to death.
With regular heroin use, tolerance to the drug develops. Once this happens, the abuser must use more heroin to achieve the same intensity. As higher doses of the drug are used over time, physical dependence and addiction to the drug develop.
Source: Drug Enforcement Administration, Drugs of Abuse (2015 Edition): A DEA Resource Guide. View here.