What are possible side effects of antihistamines?

Antihistamines, commonly used to treat allergic reactions and conditions like hay fever, block the action of histamine in the body, which helps alleviate symptoms. However, they come with a range of side effects due to their effects on the central nervous system and other systems.

Drowsiness is a well-known side effect of many first-generation antihistamines. This occurs because these medications can cross the blood-brain barrier and impact neurotransmitters involved in sleep regulation. Dry mouth is another common side effect, resulting from antihistamines' anticholinergic properties that reduce saliva production.

In contrast to drowsiness and dry mouth, the other side effects listed are not typical for antihistamines. While hyperactivity may be observed in some individuals, particularly in children, it is more characteristic of certain stimulant medications rather than a primary effect of antihistamines. Increased appetite and weight gain are generally not associated with these medications in the same way as they can be with other classes of drugs, such as certain antidepressants. Fatigue and fever are also not commonly linked to antihistamines, with fatigue potentially stemming from drowsiness but not fever.

Thus, the correct answer captures the most common and recognized side effects of antihistamines, establishing