Which symptom is least likely to be associated with the use of diphenhydramine?

Diphenhydramine is an antihistamine commonly used for its sedative effects, particularly in the treatment of allergies and as a sleep aid. Its anticholinergic properties contribute to various side effects. The sedative effect is one of the most well-known and expected outcomes when using diphenhydramine, making sedation a common symptom.

Dry mouth is another frequent side effect associated with diphenhydramine and other medications with anticholinergic activity due to their impact on saliva production. Constipation can also occur, as anticholinergics can reduce gastrointestinal motility, leading to less frequent bowel movements.

While diphenhydramine can cause various side effects, an increased heart rate is less likely to be associated with it. Antihistamines like diphenhydramine generally do not typically stimulate the heart; rather, they may lead to changes in heart rhythm in some cases but are more often associated with a calming or sedating effect. Therefore, an increased heart rate, while it may occur due to other factors or in sensitive individuals, is least likely to be a common symptom of diphenhydramine use when compared to sedation, dry mouth, and constipation.