There are a couple of studies documenting this. Interestingly even though the treatment is seen as expensive ( diacetylmorphine, security, heath staff ect ) money is actually saved due to i.e less days imprisonment, days spent in hospital, less visits to GP etc. Not least documented improvement of quality of life for the users of the site.
To my knowledge not a single death is registered in the Heroin clinics in Europe to date.