Antihistamine Side Effects

Antihistamines are commonly prescribed to treat symptoms caused by allergies. They are considered relatively safe even for self-treatment, which is why they are readily available as over-the-counter medication. There are, however, antihistamine side effects that every patient must be aware of. While some people may experience mild symptoms, such as bad breath or fatigue, others may suffer more. In rare instances, some side effects may even require the intervention of a doctor. Here are some of the most common antihistamine side effects and why they occur:

Drowsiness and fatigue

The most common antihistamine side effects is drowsiness and is quite common with medications such as chlorphenamine and promethazine. They are, in fact, formulated to help the patient fall asleep. The ingredient that causes drowsiness and a feeling of general tiredness is diphenhydramine, which not only blocks histamine receptors but also acetylcholine receptors. Antihistamine side effects can often be traced to acetylcholine, a type of neurotransmitter that activates the muscles and the nervous system. Once the reuptake is blocked by diphenhydramine, the system is flooded by more acetylcholine which then causes drowsiness. This is one of the most known antihistamine side effects that allow the medication to be used to treat insomnia.


Also common antihistamine side effects are headaches. Antihistamines can cause vasoconstriction, a condition where blood vessels contract, leading to narrowed veins and arteries. Blood flow is then restricted, which can lead to headaches.


An important antihistamine side effects to watch out for is hypertension. Because the chemical can lead to constriction of blood vessels, it could aggravate blood pressure in people who have hypertension or are borderline hypertensive. Antihistamine side effects such as this make it dangerous for patients to overdose on the medication.

Difficulty in concentrating

Antihistamine side effects can also include difficulty in focusing due to the inability of some nerve functions to perform properly. Blurry vision, drowsiness and dizziness only make this side effect worse.

Stomach upset

Stomach upset, stomach cramps and acidity are also common antihistamine side effects. The blocking of acetylcholine prevents the nerve fibers of the body’s nervous system to work normally, which interferes with the involuntary functions of the smooth muscles in the stomach. Antihistamine side effects such as these make it difficult to eat or be comfortable.

Antihistamine side effects on bodily functions

Some antihistamines are anticholinergic, blocking the action of the neurotransmitter acetylcholine. As a result, some individuals may experience antihistamine side effects that involve the slowing of normal bodily functions, including urination. The same side effect could also lead to blurred vision, dizziness and constipation. These medications also decrease the production of saliva, which causes dry mouth.

Although certain antihistamine side effects are temporary in nature and may be avoided by changing brands or antihistamine type, any discomfort or unexpected symptoms should be discussed with a medical professional in order to prevent a health emergency. If antihistamine side effects include irregular heartbeat, vomiting, nausea, shaking, seizures and worsening vision in persons with glaucoma, it’s important to seek medical help right away.