Answer: 20 mg
Step-by-step explanation:
Lethal overdose can be defined as the dose which is an indication of the lethal toxicity of the given substance. It can be defined as the dose of substance above which it is considered lethal.
The lethal dose for heroin is 20 mg above which it can be deadly for a number of people.
There are chances that some people might not die because the effect varies from person to person.