What are the possible side effects of antihistamines?

Antihistamines are commonly used to alleviate symptoms associated with allergies, such as sneezing, itching, and runny nose. One of the most notable side effects of many first-generation antihistamines is sedation or drowsiness, which occurs because these medications can cross the blood-brain barrier and affect central nervous system activity. Dry mouth is another common side effect, resulting from the anticholinergic properties of these medications that reduce saliva production. Additionally, dizziness may occur as a result of the central nervous system effects or due to reactions that generalize to functions of balance and coordination.

While some other side effects can occur with different medications, they do not specifically apply to antihistamines in the same manner or are less characteristic of the class. For example, the increased heart rate and nausea mentioned in the first option are not typical of antihistamines and are more relatable to other classes of medications or specific situations. Similarly, headache, fatigue, and insomnia are not directly linked to antihistamine use but can occasionally be experienced through various indirect pathways. The last option, rash, swelling, and fever, are more indicative of allergic reactions or side effects from other drug classes rather than the predictable outcomes of antihistamines.

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