Tulsi appears to be relatively safe when used for up to 3 months, although thorough safety analyses have not been done. The most commonly reported side effects are nausea and diarrhea.[1][2]
It’s not entirely clear whether tulsi is safe during pregnancy or when trying to get pregnant, but research in animal models suggests caution is warranted. Negative effects on the fertility of male (reduced sperm count) and female (reduced frequency of ovulation) animals have been observed. Additionally, when given during the first few weeks of gestation in female rats, tulsi leaf extract reduced the rate of implantation of the fertilized egg, reduced the number of live fetuses, and reduced fetal growth. However, it’s important to note that the dosages used in these studies ranged from 200 to 1,000 mg per kilogram of body weight, far exceeding dosages used in humans.[3][4][5]