Holy basil, also called tulsi in Ayurvedic medicine, is viewed as a relatively safe, aromatic herbal remedy popular in alternative medicine. Herbalists extol the plant's valuable healing properties and recommend it as a treatment for a range of ailments. Unlike the basil found in most Western kitchen and herb gardens, holy basil is usually only available in health food stores or online in the United States. But, like penicillin or peanuts, holy basil can cause allergic reactions or have adverse effects that are sometimes quite serious.
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People who are already prescribed a blood-thinning medication, such as heparin or warfarin, by their physician should be aware that holy basil is a mild blood-thinner. The herb intensifies the blood-thinning properties of these prescription drugs and could lead to bruising or prolonged periods of bleeding. Anyone who has a blood-clotting disorder should consider this potentially serious side effect of holy basil before ingesting the herb.
Studies show holy basil can cause a decrease in fertility in both lab animals and humans. Wellness.com cautions that holy basil may have sperm-blocking properties. Use of the herb should be avoided if this is a concern.
Low Blood-sugar Levels
One reason people choose to use holy basil is because it lowers glucose levels in the blood. Problems arise, however, when diabetics or hypoglycemics who are already taking medication to regulate or lower blood glucose levels supplement these medications with holy basil, which can significantly lower blood sugar levels even more. If blood glucose levels drop too radically, people with diabetes can experience dizziness, fainting and even shock, according to the Right Health website.
Uterine Contractions in Pregnant Women
Pregnant or breastfeeding women should not take holy basil in any form, since the herb can precipitate contractions of the uterus. The Blue Shield of California website says that pregnant and lactating women should not take holy basil, since studies are unclear about the herb's possible short- or long-term effects on both mother and baby.
Interactions with Other Herbal Supplements
Because of the way holy basil is processed in the liver, the herb can interfere with the way other herbal supplements are absorbed in the body. Holy basil can intensify the effects of certain herbs or supplements, resulting in excessive levels of those herbs in the blood.