Antihistamine medications are a common over-the-counter solution for allergy symptoms like sneezing, itching, and watery eyes. While generally considered safe, a frequent question involves their potential influence on the cardiovascular system. Specifically, people often wonder if antihistamines can increase heart rate or lead to palpitations. The effect on the heart depends heavily on the specific type of antihistamine and the individual’s underlying health.
Understanding How Antihistamines Work
Antihistamines target the effects of histamine, a chemical released by the immune system during an allergic reaction. Histamine binds primarily to H1 receptors, triggering classic allergy symptoms like inflammation, itching, and increased mucus production. The medication competitively blocks these H1 receptors, preventing histamine from binding and reducing the allergic response.
H1 receptors are found throughout the body, including the central nervous system (CNS), smooth muscles, and blood vessels. First-generation antihistamines, such as diphenhydramine, are non-selective and easily cross the blood-brain barrier. Acting on the CNS, they commonly cause side effects like drowsiness and sedation. These drugs also interact with other receptor types, contributing to a wider range of potential side effects.
Second-generation antihistamines, including cetirizine and fexofenadine, are more selective for peripheral H1 receptors outside the brain. This selectivity prevents them from easily penetrating the blood-brain barrier, resulting in less sedation. By limiting activity to the body’s periphery, these newer medications generally have a cleaner side effect profile. This difference in selectivity is fundamental to understanding their varying impacts on heart rhythm.
Why Certain Antihistamines Affect Heart Rhythm
The potential for antihistamines to affect heart rate is primarily linked to older, first-generation compounds. These drugs possess anticholinergic properties, meaning they block acetylcholine, a neurotransmitter that helps regulate the heart. By blocking muscarinic acetylcholine receptors, these medications can interfere with the normal pacemaker rhythm. This interference can lead to an increased heart rate (tachycardia) and potentially other rhythm disturbances.
Diphenhydramine, a common first-generation drug, is a potent anticholinergic that can directly cause an increased heart rate and dry mouth. This effect results from the drug’s non-selective nature, causing it to interact with receptors other than H1. In high doses or sensitive individuals, CNS stimulation caused by some first-generation drugs can also indirectly contribute to a faster pulse.
A different mechanism of cardiotoxicity involves the blockade of potassium channels in the heart, particularly the hERG channels. Certain second-generation antihistamines, such as terfenadine and astemizole, were withdrawn from the market because they blocked these channels, which are responsible for the heart’s electrical repolarization phase. This blockade can prolong the heart’s QT interval, increasing the risk for a serious arrhythmia called Torsades de Pointes. However, the second-generation antihistamines currently available (cetirizine, loratadine, and fexofenadine) were developed to lack this hERG channel-blocking ability at therapeutic doses, making them safer for the heart.
Recognizing Personal Risk Factors and Safety Measures
While most modern antihistamines are safe for the heart, certain individuals face a higher risk of adverse cardiac effects. People with pre-existing heart conditions, such as arrhythmias, inherited long QT syndrome, or high blood pressure, should exercise caution. Older adults are also more vulnerable to adverse effects from first-generation antihistamines due to changes in drug metabolism and increased sensitivity to anticholinergic effects.
A safety concern arises when antihistamines are combined with other substances, especially decongestants like pseudoephedrine (often denoted by a “D” in the product name). Decongestants constrict blood vessels, which can directly increase blood pressure and heart rate. This is hazardous for those with underlying cardiovascular issues. Combining certain antihistamines with drugs that inhibit liver enzymes, such as antifungals or macrolide antibiotics, can raise the antihistamine concentration in the bloodstream, increasing the potential for toxic side effects.
To ensure safe use, read the medication label carefully and choose a non-sedating, second-generation antihistamine without a decongestant if heart health is a concern. Before starting any new over-the-counter medication, especially with an existing heart condition or while taking other prescription drugs, consult a healthcare provider. If you experience a rapid or irregular heartbeat, palpitations, or dizziness after taking an antihistamine, seek medical attention immediately.