As a medicinal herb it is used to prevent colds and flu, and cleanse the respiratory system of infection. The oil is used as an antioxidant, relieving aches and pains. A mixture of leaves and seeds, with black pepper (Piper nigrum) is given to pregnant women suffering from malaria. Fresh flowers are used to treat coughs and colds. The plant has shown promise in clinical trials in alleviating hepatic dysfunction, and is an ingredient of pills and ointment to cure eczema. The essential oil from some populations of holy basil contains high levels of eugenol. In traditional Thai medicine, the leaf or whole plant is used to alleviate nausea, vomiting, diarrhoea and flatulence. Alcohol extracts from the plant heal peptic ulcers.

Synonym = Ocimum sanctum

The leaf extract is effective in checking the spread of the fungal pathogens Pyricularia oryzae and Rhizoctonia solani, which cause blast disease and sheath blight disease of rice. Antibacterial activity and deterrent against the larvae of root knot nematode (Meloidogyne incognita) have been reported, and the oil is a mosquito repellent. Ocimum tenuiflorum is used as a major flavouring ingredient in Thai cuisine. In India, Hindus grow Tulsi as a religious plant in their homes and temples and use its leaves in routine worship. Because it is regarded as sacred in Hinduism, it is used to make rosaries - the woody stems are used to make the beads.

Ocimum gratissimum contains citral a monterpene alkaloid. There was an increased incidence in malignant lymphomas in female mice, but not in rats or male mice. No definite evidence of genotoxicity found though 'positive results in sister chromatid exchange assay.' Ocimum basilicum contains estragole and safrole, alkylbenzenes, which are carcinogenic and genotoxic in rodents.