AimThe main aim of this study was to investigate the additional effects of L‐theanine, an amino acid in tea and an analog of glutamate with neuroprotective and anti‐depressant properties, on obsessive–compulsive disorder (OCD) symptoms in combination with fluvoxamine.MethodsPatients from either sex aged between 18 and 60 years diagnosed with OCD, based on the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM‐5), who had a Yale‐Brown Obsessive Compulsive Scale (Y‐BOCS) score of more than 21 were enrolled in a double‐blinded, parallel‐group, placebo‐controlled, clinical trial of 10 weeks to receive either L‐theanine (100 mg twice daily) and fluvoxamine (100 mg daily initially followed by 200 mg daily after week 5) or placebo and fluvoxamine. The primary outcome of interest in this study was the Y‐BOCS total score decrease from baseline.ResultsFrom a total of 95 evaluated patients, 50 completed our study; 30 were randomly assigned to each group. Multivariate analysis (ANOVA) showed a significant effect of time treatment for L‐theanine in obsession subscale (F = 5.51, P = 0.008) of the Y‐BOCS score but not in the total and compulsion scores. Our results showed significantly more improvement in obsession subscale scores in L‐theanine compared to placebo group (P = 0.007, Cohen's d = 0.82). Also, total Y‐BOCS scores were lower in L‐theanine compared to placebo group at week 5 (P = 0.039, Cohen's d = 0.60) and 10 (P = 0.008, Cohen's d = 0.80). However, there was no significant between‐group differences in compulsion subscale scores. Complete response was also more frequent in the L‐theanine group (P = 0.0001).ConclusionFindings in this study suggest L‐theanine as a relatively safe and effective adjuvant therapy for moderate to severe OCD.

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