Is tulsi (holy basil) safe during pregnancy?

The short answer

The WHO monograph lists pregnancy and breastfeeding as contraindications for holy basil leaf because feeding the leaf or its extracts to rats and mice prevented embryo implantation and reduced fertility. Those are animal findings at high doses, and no human pregnancy outcome studies exist, so the honest position is uncertainty rather than proven harm. Avoid teas taken as a daily remedy, powders and extracts while pregnant, breastfeeding or trying to conceive; ordinary culinary amounts in a Thai dish are a different exposure but still deserve a conversation with your midwife or clinician.