Which of the following is NOT commonly associated with drugs that cause saliva reduction?

Caffeine is not commonly associated with saliva reduction compared to the other options listed. Antidepressants, antihistamines, and antihypertensives are well-documented for their side effects that include xerostomia, or dry mouth, which is characterized by a reduction in saliva production.

Antidepressants often affect neurotransmitters that can influence salivary gland function, leading to decreased saliva output. Antihistamines, used primarily for allergies, can have an anticholinergic effect, which interferes with the body's ability to produce saliva. Antihypertensives can also lead to dry mouth as a side effect, particularly those that work on the nervous system or those that are diuretics.

Caffeine, however, does not have a significant impact on saliva production. In fact, it can stimulate the central nervous system and may even promote salivation to some extent. Therefore, it is distinct from the other drug classes mentioned, making it the correct answer to the question.