In Hong Kong, the percentage of older drug users has increased over the last two decades. However, the motivations behind their drug-use behaviours have received little research attention. This study focuses on older drug users who are enrolled in methadone treatment programmes but still use illicit drugs (mixed use). Some studies in the criminological literature and government discourse consider drug users to be passive and lacking self-control. However, in-depth interviews in with 25 older mixed users (aged over 50 years) in Hong Kong revealed that mixed use is one of the various strategies they actively employ to improve their self-perceived quality of life. Using the framework of the selective optimization with compensation model, this study (1) describes the strategies older mixed users adopt as active agents to improve their self-perceived quality of life while coexisting with their addiction; and (2) explains how these strategies were affected and constrained by Hong Kong’s prohibitionist drug policy. I infer that prohibitionist drug policies that emphasize on total drug abstinence may fail to cater to the needs of older drug users who have undergone several relapses and treatments in their lifetimes and do not think they can give up using drugs. This study also provides evidence to show how some drug users may act as active agents to manage and coexist with their addiction, and their agency seems to be constrained by the wider drug policy implemented in Hong Kong.