On the basis of the inclusion and exclusion criteria described above, the potential candidates for participation in the trials were recruited in a selection procedure which, in broad terms, involved the following steps:
1. pre-selection of a source population on the basis of the available information from the methadone treatment registration system;
2. patients in the source population were invited in a random order to an initial meeting with the physician of the local treatment unit; subsequent screening on various selection criteria and provision of oral and written information;
3. candidates who met the initial selection criteria and who provided initial informed consent entered the qualification period; an initial screening took place in which all selection criteria were examined;
4. four to eight weeks later a final screening was conducted among the candidates who had passed the initial screening; during this final screening, the most important selection criteria were again assessed, and final informed consent was obtained;
5. randomization took place with all patients who met all inclusion criteria and were not barred by any of the exclusion criteria at the final screening.

Step 1. Registration system
In the first selection step, the available information from the methadone maintenance treatment registration system was examined, with regard to the following selection criteria:

• heroin dependency of at least five years;
• registered in a methadone program and regular contact with the program;
• minimum daily methadone dose level of 50 mg (inhaling) or 60 mg (injecting) for at least four consecutive weeks in the previous five years;
• at least 25 years old;
• at least three years registered in the city area of the treatment site.
  

Step 2. Initial meeting
All patients who met the criteria described above received a randomly assigned patient identifier number, and were invited to an initial meeting with the physician of the local treatment site in the order of their identifier number. During this meeting, the patient received extensive oral and written information about the objectives and procedures in the study, the potential benefits and risks for the patient, and the expected co-operation during the study. In addition, the following selection criteria were examined:

• daily or nearly daily use of illicit heroin;
• the use of heroin through intravenous injection or inhalation;
• the ability to understand the Dutch language;
• the ability and willingness to participate in the study according to the requirements (e.g. life expectancy, visits required for the assessments, use of the prescribed heroin in the treatment site, an anticipated incarceration of no longer than three months).
  

Step 3. Initial screening
Patients who met the criteria described in step 2 and who were not barred by exclusion criteria were referred to the local research team and the physician for an initial screening assessment in the four to eight weeks qualification period of the study. Prior to the screening assessment, the physician obtained initial written informed consent from the patient. During this initial screening, the following selection criteria were examined:

• Dutch nationality or legally residing in the Netherlands;
• examination of the entry criteria concerning poor physical health, and/or mental health, and/or social functioning (see paragraph 2.1.2):
  - MAP-HSS score 8;
  - SCL-90 score 41 (male) or 60 (female);
  - at least six days of illegal activities in the previous month;
  - at least six days in the previous month without personal contact with a non-drug-using person;
• medical screening:
  - contra-indication for health reasons;
  - pregnancy or continued lactation;
  - life expectancy shorter than one year;
• contra-indication for aggressive behavior;
• otherwise not able to participate in the study (including expected incarceration three months);
• voluntary heroin abstinence two months in the previous year;
• requiring a daily dose of more than 150 mg of methadone and/or 1000 mg of heroin;
• participation in another clinical trial, directed at addictive behaviors.

Step 4. Final screening (baseline assessment)
Four to eight weeks after the initial screening, patients who met the criteria described in step 3 and who were not barred by exclusion criteria received their final screening assessment. Prior to the final screening, patients were asked to provide final written informed consent. The following selection criteria were examined:

• final informed consent;
• daily or nearly daily use of illicit heroin;
• examination of the entry criteria concerning poor physical health, and/or mental health, and/or social functioning:
  - MAP-HSS score 8;
  - SCL-90 score 41 (male) or 60 (female);
  - at least six days of illegal activities in the previous month;
  - at least six days in the previous month without personal contact with a non-drug-using person;
• medical screening (including psychiatric evaluation):
  - contra-indication for health reasons;
  - pregnancy or continued lactation;
  - life expectancy shorter than one year;
• heroin addiction of secondary importance compared to an existing non-opiate dependency.
  

Step 5. Randomization
Within three to four days after the final screening (baseline assessment), all patients who had provided informed consent, met all inclusion criteria, and who were not barred by any of the exclusion criteria were randomized to one of the treatment conditions. Within days following the outcome of the randomization procedure, all patients were orally informed by the local treatment staff about the treatment condition they were randomized to.