Hawthorn (Crataegus monogyna) fruits
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Hawthorn berry is an herb that has been used for centuries for its health benefits. According to the University of Maryland Medical Center, it is a treatment for respiratory and circulatory disorders, as well as high blood pressure. Hawthorn is an herb that has few side effects, but because of its ability to effectively treat heart and blood pressure problems, the side effects that do occur are often related to those two systems.

Heart Side Effects

Because hawthorn is so effective in treating many heart issues, it can also cause side effects if taken by those who do not need it. While it is used to treat heart palpitations, one of the side effects can be an increase in heart palpitations. According to Memorial Sloan-Kettering Cancer Center, the palpitations, or increased heart rate, is often a sign of overdose. Other adverse reactions involving the heart are increased chest pain and fatigue during activity.

Digestive Side Effects

Reports of digestive trouble are occasionally reported with hawthorn use. A review on hawthorn berry in a 2006 issue of "Drug Safety" reports that nausea was one of the most commonly noted side effects, along with dizziness and intestinal hemorrhaging.

Medication Interaction Side Effects

The report in "Drug Safety" also states that the few negative side effects that are reported with hawthorn berry are often due to using the herb "unsupervised." This is especially the case if hawthorn is used while you are currently taking a prescription for treatment of heart problems or blood pressure. The UMMC reports that hawthorn berry can interfere with the effects of two medications in particular: digoxin and phenylephrine. Digoxin is used to treat irregular heartbeats, and hawthorn can interfere with, or adversely increase the effects of, this medication. Phynylephrine tightens blood vessels, and is found in many over-the-counter medications, and hawthorn expands blood vessels, which can render phynylephrine medications ineffective. Due to its action of relaxing blood vessels and increasing blood flow, if you are on medication to reduce blood pressure, using hawthorn can cause your blood pressure to go too low.