Anyone can overdose: first time users, long-time users, people using prescription medication, old people, young people, people being released from jail or treatment, etc. There is no exact formula for determining how much of a certain drug or combination of drugs, will lead to an overdose. 

An individual’s physical characteristics play a role: weight, health, tolerance for a drug at that particular time, drug potency, route of administration, frequency/amount of use or how much they have eaten that day.