Illegal drug use is a hidden phenomenon, making it extremely difficult to obtain accurate estimates of untreated use. Yet it is this very estimate of incidence that is required by policy makers and service providers who are responsible for allocating increasingly scarce resources in times of worldwide recession. Using Irish data on more than 6,000 cases of clients presenting for first treatment, a Gamma incubation period distribution describing progression to first opiate treatment is fitted. An analytical solution of the back calculation method and a linear Volterra integral equation are used to project back and estimate the hidden, untreated population of opiate users.