Which characteristic is true regarding the administration of antihistamines?

Antihistamines are commonly used to alleviate symptoms associated with allergic reactions by blocking the effects of histamine, a chemical released during such responses. The effectiveness of antihistamines is highest when administered before the onset of symptoms because they prevent histamine from binding to its receptors as it is released during an allergy attack. By taking antihistamines proactively, individuals can mitigate or even avoid symptoms such as sneezing, itching, and runny nose.

This preventative approach is due to the mechanism of action of antihistamines, which is to inhibit the receptor sites for histamines rather than attempting to displace histamines that have already activated these receptors. When symptoms are already present, the impact of antihistamines may be less effective, as they can't reverse the effects of histamine that are already in place. Therefore, the timing of administration is critical for maximizing their benefits.