Benign prostatic hyperplasia (BPH) is one of the most prevalent human diseases and a major cause of lower urinary tract symptoms (LUTS). Some men respond to current medical treatment (mainly α-1 adrenoreceptor antagonists and 5 α-reductase inhibitors), but a large proportion of patients continues to need a surgical procedure to treat resistant LUTS or even more serious complications of BPH, creating the emerging necessity for novel pharmacological therapies.

Oxitriptan may have a possible positive effect on BPH associated symptoms with probably no impact in sexual function (which is a common side effect of the current drugs for BPH associated symptoms). Also, improvement in symptoms could be higher than that of current drugs used for this condition.

This is a single-center parallel group, randomized clinical trial. The study will take place in Hospital de Braga (Urology department). Eligible patients will be randomized to receive tamsulosin 0.4mg (once a day, q.d.) or 5-HTP (5-hidroxitriptophan) 100mg (three times a day, t.i.d.), for 6 months.

Inclusion Criteria:

• Written informed consent;
• Male patients with BPH for which tamsulosin is the therapeutic option per SoC;
• Aged ≥50 and less than 75 years old;
• With prostate volume ≥30 cm3 by TRUS;
• Diagnosed with LUTS defined by a stable IPSS total score ≥13 points.

Exclusion Criteria:

• Patients with post-void bladder residual volume ≥250 ml;
• Patients with intravesical obstruction from any cause other than BPH;
• History of any procedure considered an intervention for BPH;
• Patients with active urinary tract infection;
• History of recurrent urinary tract infections;
• Current prostatitis or diagnosis of chronic prostatitis;
• History of prostate or invasive bladder cancer;
• Use of 5 α-reductase inhibitors within 6 months;
• Phytotherapy within 2 weeks before entry;
• Use of serotonin reuptake inhibitors or monoamine oxidase inhibitors;
• Patients with acute or chronic kidney failure;
• Patients with diagnosed or suspicion of intolerance to lactose;
• Patients submitted to general anesthesia in the past 4 weeks;
• Known intellectual disability that may hampers giving informed consent and would make the patient inappropriate for entry into this study, in the judgment of the investigator.

Complete (double) blinding is not possible to achieve, nonetheless, in order to lessen assessment bias, efficacy and safety assessment procedures will be performed by the assigned blinded research staff. In order to assure blinding across the study, randomization data will be kept confidential and will not be accessible to anyone, except to unblinded research staff, pharmacy staff. Both groups will be monitored for the same study duration.