The primary objective of this study is to determine whether favorable response to naltrexone orally taken in treatment of GD can be predicted by patterns of visual scanning, assessed by eye-tracking technology before, at the start and throughout gambling treatment with naltrexone.

The study will be a double blind 12-week controlled trial with two groups, one taking active drug (naltrexone) and the other receiving placebo. Patients on both groups will be assessed for GD symptoms and severity, major comorbid psychiatric disorders and related psychopathology (impulsivity, craving and locus of control), and eye-tracking patterns prior to and one hour after the administration of the first dose, one week after and at treatment completion.

On the active drug group, naltrexone will be prescribed on flexible dose mode, receiving 50mg per day on the first week, with an increase of 25mg each week until a maximum of 200 mg per day or as much as tolerated by the patient.

On the first day of evaluation patients will first be interviewed to check GD diagnosis and screen other medical conditions that may exclude patients from the trial. The selected individuals will be randomly assigned to either naltrexone or placebo group; After that they will answer several questionnaires and have the first eye tracking assessment. Each individual will receive either placebo or naltrexone and be assessed 1 hour after drug administration.

After the first week patients will have the third eye tracking assessment, as well as several questionnaires On the next weeks patients will be constantly monitored for side effects and gambling symptoms.

On the twelfth week patients will have the fourth and last eye tracking assessment During the 12-week trial both groups will have psychoeducational sessions, on weeks 2,4,6 and 8. During sessions patients have access to audio-visual material and receive Self-help material, this intervention is based on Hodgins proposal (Hodgins DC et al, 2005).

Adherence will be verified by counting pills consumed by the patient. In the weeks without face-to-face medical assessment, patients will be evaluated by telephone interview to monitor adverse effects of medication. If the evaluator identifies a worrying adverse reaction, the patient will be advised to anticipate the face-to-face interview.

Inclusion Criteria:

• The inclusion criteria are:

  1. Men and woman aged 21 to 60 years;

  2. Female patients should be:

     • postmenopausal for at least one year, or;
     • are surgically unable to become pregnant (undergoing bilateral hysterectomy or oophorectomy or tubal ligation or otherwise unable to become pregnant), or;
     • be practicing an acceptable method of birth control (defined as hormonal contraceptives, spermicide plus barrier, a single vasectomized partner and / or intrauterine device);

  3. Have read and signed the informed consent form.

Exclusion Criteria:

• 1. Hypersensitivity to naltrexone or contraindication of naltrexone use; 2. Exposure to another pharmacological drug in the last 30 days; 3. Pregnancy or lactation; 4. Kidney dysfunction: Creatine serum > 133 mmol/L in men > 124 mmol/L in women, which correspond > 1,51 mg/dL e > 1,41 mg/Dl; 5. Liver dysfunction (aspartate transaminase (AST) and alanine transaminase (ALT) > 2times the upper limit of normal; 6. Cardiovascular disease, hypertension; 7. Lifetime history of bipolar disorder, Obsessive compulsive disorder, schizophrenia or any psychotic disorder, or depression (BDI> 30 points), clinically significant suicidality; 8. Lifetime history of drug (except nicotine) or alcohol; 9. Hematologic or immunologic dysfunction; 10. Subjects receiving psychoactive drugs, except sporadic use of benzodiazepines; 11. Simultaneous participation in other GD-related; 12. Lack of reliable contact information; 13. Illiteracy of other condition that difficult reading and understanding the study questionnaires and orientations; 14. Not having a cellphone line;