Which of the following is a desired effect of antihistamines?

Antihistamines are primarily used to manage allergic reactions and their symptoms. When an allergic reaction occurs, the body releases histamine, a substance that contributes to symptoms like itching, sneezing, runny nose, and swelling. The primary desired effect of antihistamines is to block the action of histamine at its receptor sites, thereby stopping or reducing the manifestations of allergic reactions. This can be extremely beneficial for individuals suffering from conditions such as hay fever, hives, or allergic rhinitis, as it helps alleviate discomfort and improves overall quality of life.

The other options do not align with the primary function of antihistamines. For instance, increasing mucus production would not address allergies and could contribute to more discomfort. Enhancing drug absorption is not a function of antihistamines; rather, it pertains to the pharmacokinetics of how drugs are processed in the body. Lastly, while some antihistamines have sedative effects, they are not intended for relieving muscle pain, which typically would require pain relief medications rather than antihistamines.