The majority of overdose deaths occur after a period of abstinence (getting clean, running out, etc.) followed by using the same amount (or more in the case of withdrawal) of the drug before the cessation. It can take as little as 48 hours for your tolerance to drop by a meaningful margin.

My ex-girlfriend died in September of this year from a heroin overdose. She was in the process of getting clean and then relapsed.

http://www.businessinsider.com.au/philip-seymour-hoffman-ove...

https://www.thefix.com/content/high-risk-relapse-how-going-b...