Basil is possibly safe when used by mouth for short periods of time, up to six weeks. It is not known if long-term use is safe.

Special Precautions & Warnings:

Pregnancy and breast-feeding: Not enough is known about the use of basil during pregnancy and breast-feeding. Stay on the safe side and avoid use.

Surgery: Basil might slow blood clotting, so there is a concern that it could increase the risk of bleeding during and after surgery. Stop using basil at least 2 weeks prior to a scheduled surgery.

(Source: www.webmd.com)