This study evaluates the efficacy of NEUROPHARMAGEN pharmacogenetic test in the selection of the pharmacological treatments for patients with Major Depressive Disorder (MDD), both with and without psychiatric comorbidities. Patients will be randomly asigned to test-guided treatment prescription or to treatment as usual ina a 1:1 ratio; the results of the test will not be disclosed to the later until the end of the 3-month follow-up period. The study will compare the rate of treatment responders among both groups, based on patient-reported improvement collected by blind telephone interview.

NEUROPHARMAGEN is a genetic test developed by AB-BIOTICS S.A. that enables the specific analysis of Single-Nucleotide Polymorphisms related to the pharmacokinetics and pharmacodynamics of different psychoactive drugs. The aim of the test is to provide the psychiatrist with information that can help him/her identify the most suitable medication for each patient.

In the study group, the psychiatrist will have the results of the NEUROPHARMAGEN test as supporting information to help him/her select the best treatment for the patient. In the control patient group, the treatment will be selected and prescribed in accordance with routine clinical practice.

This is a naturalistic, double-blind, randomized, multicentric clinicaltrial carried out in Spain at psychiatry departments of several public hospitals. The study aims to include a total of 520 patients with MDD, including patient with significant psychiatric comorbidities such as anxiety or substance abuse.

The study will compare groups based on the rate of treatment responders, defined as a score of 2 or less (i.e. "Much better"/"Very much better") in the Patient Global Impression of Improvement scale (PGI-I). This scale will be collected by blind telephone interviewers, so as to have a double-blind assessment (patient and interviewer).

Inclusion Criteria at pre-randomization visit:

• Patients with diagnosed for major depressive disorder according to DSM-IV-TR criteria
• Patients who give their written informed consent to participate in the study. In the case of disabled patients, informed consent from the legal representative or responsible relative.
• Patients with a doctor-rated Clinical Global Impression - Severity Scale (CGI-S) score equal to or greater than 4.
• Patients who are diagnosed de novo who, in the doctor's opinion, require medication or are receiving treatment and require an antidepressant, antipsychotic or mood stabiliser as a replacement or additional medication

Exclusion Criteria at pre-randomization visit:

• Patients who, in the investigator's opinion, will not be able to complete the study follow-up.
• Patients who are actively taking part in or who have taken part in another clinical trial in the past 3 months.
• Patients who are pregnant or breast-feeding, or patients who plan to become pregnant within the next 12 months.
• Patients who are or who require treatment with quinidine, cinacalcet and/or terbinafine (potent CYP2D6 inhibitors).

Inclusion criteria at randomization visit:

Patients who meet the screening criteria at the pre-randomisation visit must meet the following criteria at visit 1 to be randomised. Otherwise, they will be excluded from the active follow-up phase. The criteria are:

• Patients with a PGI-I score of 4 or more.
• Patients with a CGI score of 4 or more.
• Patients whose dose of pharmacological treatment, in the doctor's opinion, requires suppression, replacement, addition or modification with an antidepressant, antipsychotic or mood stabiliser.