Objective Vitamins B6 and B12 are involved in metabolic processes that decrease neural excitation and increase inhibition. This double‐blind study investigated the effects of supplementation for 1 month with a high‐dose of B6 or B12, compared to placebo, on a range of behavioural outcome measures connected to the balance between neural inhibition and excitation. Methods 478 young adults were recruited over five linked phases. Self‐reported anxiety (N = 265) and depression (N = 146) were assessed at baseline and after supplementation. Several sensory measures acted as assays of inhibitory function and were assessed post‐supplementation only; these were surround suppression of visual contrast detection (N = 307), binocular rivalry reversal rate (N = 172), and a battery of tactile sensitivity tests (N = 180). Results Vitamin B6 supplementation reduced self‐reported anxiety and induced a trend towards reduced depression, as well as increased surround suppression of visual contrast detection, but did not reliably influence the other outcome measures. Vitamin B12 supplementation produced trends towards changes in anxiety and visual processing. Conclusions Our results suggest that high‐dose Vitamin B6 supplementation increases inhibitory GABAergic neural influences, which is consistent with its known role in the synthesis of GABA.

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