Previous studies of concentrations of serotonin reuptake inhibitors (SRIs) versus therapeutic efficacy have yielded inconsistent results. Even if the relationships between the individual's serotonergic system and the clinical symptoms of obsessive-compulsive disorder (OCD) are poorly understood, the SRIs are consistently effective in OCD. However, studies on SRI concentrations in OCD treatment are rare.

To identify possible links between paroxetine concentrations and anti-obsessive response.

In a randomised, double-blind trial, comparing clomipramine, paroxetine and placebo in OCD treatment, serum paroxetine levels were measured after 1 week and after 4 weeks of treatment in 18 patients. Anti-obsessive response was assessed with Yale-Brown obsessive compulsive scale (Y-BOCS) and patients’ global evaluation (PGE), after 12 weeks of treatment.

Serum paroxetine concentrations after 4 weeks suggested a therapeutic interval between 50 and 240 nmol/L (13–63ng/mL). The mean Y-BOCS decrease was 54% inside versus 7% outside this interval (t=3.96; P=0.0011).

Paroxetine levels seemingly predicted clinical outcome. Studies with a greater number of patients are necessary in order to confirm this finding and to discern whether it is useful in clinical practice.