Herbal teas may appear innocuous, especially if they contain common cooking herbs such as parsley, which has no harmful effects when used in normal quantities in foods 1. But because herbal teas contain a more concentrated form of the herb, they can have harmful effects if you drink them in large enough quantities.

Fluid Loss

The oil found in the roots and leaves of parsley contains two substances, myristicin and apiol, which can have a diuretic and laxative effect 1. Drinking large amounts of parsley tea could lead to potassium and sodium loss from diuresis, researchers from the University of Beirut reported in the March 2002 "Journal of Ethnopharmacology." Parsley may also have a laxative effect due to its actions on fluid absorption from the colon, the same researchers reported in the September 2001 issue of "Phytomedicine." Drinking large amounts of parsley tea could cause fluid loss through diarrhea as well as through diuresis, possibly leading to dehydration 14.

Hallucinogenic Effects

Myristicin plays a role in the hallucinognic effects of nutmeg and may have the same effects when consumed in large amounts of parsley 1. Consuming large amounts of parsley oil can cause giddiness and loss of balance 1.

Pregnancy Effects

Parsley tea can have several negative effects during pregnancy 1. Apiol and myristicin in parsley can cause uterine contractions 1. Ingesting large amounts of these substances could cause miscarriage or preterm labor. Parley can also cause a potentially dangerous increase in the fetal heart rate and could affect your baby's hemoglobin level; hemoglobin, part of the red blood cell, carries oxygen throughout the body.

Potential Allergic Reactions

If you have an allergy to other members of the Apiaceae family, such as carrots, celery or fennel, you have an increased risk of having an allergic reaction to parsley 1. Allergic reactions could include hives, rash, difficulty breathing, chest tightness, itching or facial swelling. Report allergic reactions to your medical practitioner and don't drink large amounts of parsley tea if you have a carrot, celery or fennel allergy until you're sure you won't have an allergic reaction 1.