Tea tree oil (Melaleuca alternifolia) has been investigated for its efficacy in treating head lice (Pediculus humanus capitis). Multiple in vitro studies and a number of small clinical trials have shown that tea tree oil possesses both insecticidal and ovicidal properties, likely due to its active components such as terpinen-4-ol and 1,8-cineole. For example, a 2012 randomized controlled trial (Heukelbach et al., 2012) evaluated a combination of tea tree oil and lavender oil, finding that it was significantly more effective than placebo in eradicating lice infestations. Another study (Cummings et al., 2010) demonstrated significant in vitro efficacy of tea tree oil against both lice and their eggs.

However, it is important to note that most clinical evidence comes from small-scale studies, sometimes using tea tree oil in combination with other agents, making it difficult to attribute the results to tea tree oil alone. Additionally, while laboratory studies consistently demonstrate toxicity to lice, real-world effectiveness may be lower due to factors such as application method and concentration. Adverse reactions, particularly skin irritation and potential allergic reactions, have also been reported and should be considered.

In summary, there is moderate scientific evidence supporting the use of tea tree oil against lice, but more large-scale, high-quality trials are needed to establish its effectiveness and safety profile compared to standard treatments.