Basil contains a strong-scented volatile oil composed primarily of terpenoids, particularly eugenol, thymol, and estragole. Basil also has what are known as chemotypes, minor variations among plants that contain significantly different mixes of constituents. The exact components of basil oil vary widely, being affected not only by these chemotypes but also by factors such as the time of day of harvest.⁴ This may account for some of the variability in scientific research and reports of medicinal efficacy of basil from culture to culture.

Preliminary studies on holy basil and hairy basil have shown that the leaf and seed may help people with type 2 diabetes control their blood sugar levels.^{5, 6, 7} While the action-mechanism of the leaf is not understood, the seed may work by providing dietary fibre, which helps prevent rapid blood sugar elevations after meals. In addition, the seed has been found to relieve constipation by acting as a bulk-forming laxative in one uncontrolled human study.⁸ A similar study showed the seeds useful in elderly people who experienced constipation after undergoing major surgery.⁹

The volatile oil of basil has shown antibacterial, antifungal, and antiviral activity in test tube studies.¹⁰ It is also believed to act as a carminative, relieving intestinal gas, and as a mild diuretic, though these actions have yet to be definitively proven.¹¹