During a routine checkup, a patient states that she is unable to take the prescribed antihistamine because of one of its most common adverse effects. The nurse suspects that which adverse effect has been bothering this patient?

The correct answer is C: Drowsiness. Antihistamines commonly cause drowsiness as a side effect due to their sedative properties. This can impact a patient's ability to function normally and may lead to safety concerns. Choices A, B, and D are not typical adverse effects of antihistamines. Constipation and abdominal cramps are more commonly associated with anticholinergic medications, while decreased libido is not a common side effect of antihistamines. Therefore, C is the most likely adverse effect affecting the patient's ability to take the prescribed antihistamine.