Antihistamines are commonly used medications to manage symptoms associated with allergies. Many people wonder about the potential effects of these medications on blood pressure. The relationship between antihistamines and blood pressure is not simple and can be influenced by several factors, including the specific type of antihistamine and individual physiological responses.
How Antihistamines Affect the Body
Antihistamines work by blocking the action of histamine, a chemical compound released by the body during allergic reactions. Histamine binds to specific receptors on cells throughout the body, triggering various allergic symptoms such as sneezing, itching, watery eyes, and nasal congestion.
There are two main generations of antihistamines: first-generation and second-generation. First-generation antihistamines, like diphenhydramine and hydroxyzine, are older compounds that readily cross the blood-brain barrier. This often leads to sedation. In contrast, second-generation antihistamines, such as loratadine and cetirizine, were developed to be less sedating because they do not easily cross the blood-brain barrier. They primarily target histamine receptors located outside the brain, in the periphery of the body.
Antihistamines and Blood Pressure Regulation
Certain antihistamines, particularly those from the first generation, can influence blood pressure. Diphenhydramine and hydroxyzine are examples of first-generation antihistamines that have been associated with a potential to lower blood pressure. This effect is generally not a typical side effect, but it can occur.
One mechanism contributing to lower blood pressure is sedation. The drowsiness caused by first-generation antihistamines can lead to decreased overall physical activity, which may result in a slight reduction in blood pressure. Additionally, these older antihistamines can exhibit anticholinergic effects, meaning they block the action of acetylcholine, a neurotransmitter involved in regulating various bodily functions, including heart rate and blood pressure. Anticholinergic drugs can relax the smooth muscles in arteries, leading to vasodilation and a drop in blood pressure.
First-generation antihistamines also possess alpha-adrenergic blocking properties. Alpha-adrenergic receptors are found in vascular smooth muscles and, when activated, cause blood vessel constriction. By blocking these receptors, certain antihistamines can cause blood vessels to relax and widen, a process called vasodilation, which in turn can lead to lower blood pressure. This effect can be more noticeable when changing positions, potentially causing orthostatic hypotension, a drop in blood pressure upon standing.
Second-generation antihistamines are less likely to cause blood pressure changes. Their reduced ability to cross the blood-brain barrier means they have fewer sedating, anticholinergic, or alpha-blocking effects that might influence blood pressure. When these newer antihistamines are used alone, they are generally not known to raise blood pressure. However, it is important to note that combination products containing decongestants alongside second-generation antihistamines can increase blood pressure.
Important Considerations and Medical Guidance
Pre-existing medical conditions, such as hypertension or heart conditions, can influence how antihistamines affect blood pressure. For instance, individuals with heart problems may have a higher risk of low blood pressure when taking hydroxyzine.
Concurrent use of other medications, especially those for blood pressure, sedatives, or alcohol, can also alter the effects of antihistamines on blood pressure. For example, diphenhydramine may counteract the effects of some blood pressure medications. Combination products that include decongestants like pseudoephedrine can raise blood pressure, even if the antihistamine component alone does not. Decongestants narrow blood vessels throughout the body, which can elevate blood pressure.
It is always advisable to consult a healthcare professional before taking antihistamines, particularly if you have high blood pressure, are currently on blood pressure medication, or have any underlying heart conditions. They can provide guidance on safe options, such as second-generation antihistamines not combined with decongestants. Monitoring for symptoms like dizziness or lightheadedness is recommended, and any adverse effects should be discussed with a doctor.