The purpose is to study the relation between pupillary diameter measured under scotopic conditions, and the pain thresholds measured at the time of peak and residual effect of methadone/buprenorphine in opiate-dependent patients following a substitution program.

Patients substituted since at least 3 months shall be included during a regular control visit.

Each session shall consist in

• check for toxics in urine sample;
• measurement of pupil diameter;
• measurement of mechanical punctuate pain threshold;
• measurement of mechanical bone pain threshold.

A first session shall be done at time of residual effect of the opiate. A second session shall be done at time of peak effect of the opiate.

Inclusion Criteria:

• substitution treatment stable since at least 3 months
• capacity to understand the protocol
• likely to come to visits
• covered by French welfare

Exclusion Criteria:

• chronic pain
• concomitant acute pain
• pregnancy or breast feeding
• relevant mental disease
• peripheral neuropathy
• diabetes
• regular intake of ketamine
• neuroleptic concomitant treatment

Opiate dependent patients