Kavalactones are the primary active constituents of Piper methysticum (kava), a plant traditionally used in the South Pacific for its calming effects. Scientific research has shown that kavalactones have anxiolytic (anxiety-reducing) and sedative properties, primarily through their modulation of neurotransmitter systems in the central nervous system, including GABAergic pathways. These effects can indirectly support the parasympathetic nervous system, which is responsible for "rest and digest" functions in the body, by promoting relaxation and reducing sympathetic nervous system overactivity. Several clinical studies and reviews have found kava extract to be effective in reducing symptoms of anxiety compared to placebo, with some evidence suggesting it may help with stress-related disorders.

However, while kavalactones do not act directly on the parasympathetic nervous system, their calming and anxiolytic actions may facilitate parasympathetic dominance, thereby supporting the body's relaxation response. The evidence base includes randomized controlled trials and meta-analyses, but concerns about liver toxicity have limited its widespread use in some countries. Overall, the scientific evidence supporting the use of kavalactones for nervous system support—specifically in relation to parasympathetic function—rates as moderate (3/5), being mostly indirect but nonetheless supported by human studies on anxiety and stress reduction.