Heroin Addiction

Factual Questions
1

After reading William Burroughs ‘Junky’ I learnt that a heroin addict may have to use heroin for months and months before feeling any withdrawel symptoms but even if that person comes off heroin for years (Burroughs cites 10) then starts using again they will feel withdrawel symptoms very quickly. What is the medical reason for this? I find this hard to understand as you would have thought that withdrawel is simply a case of the drug leaving your system.