People who inject heroin have a risk of death six times higher than the general population. The most effective treatment for heroin users is the prescription of legal substitution of drugs, most commonly methadone in the Republic of Ireland. This is called opiate or methadone substitution treatment (MST). Heroin addiction has been characterized as a chronic relapsing condition, where patients cycle in and out of treatment relapsing to heroin use. Recent studies have consistently shown that patients are at greatest risk of dying when they leave treatment. Patients may leave treatment for a variety of reasons such as drug relapse, imprisonment and becoming tired of methadone treatment. This PhD project aims to understand the complexity of treatment cessation, by identify factors which lead to a patient leaving treatment and also factors which promote staying in treatment. This will be achieved through an in-depth analysis of retention in methadone substitution treatment across four studies, identifying both risk factors for treatment cessation and protective factors for retention using a mixed methods study design. The four studies to be conducted will be as follows: a systematic review of previous studies conducted an examination of trends amongst Irish MST users, and interviews of both patients and service providers to get both perspectives in relation to treatment cessation.