Which of the following medications has possible respiratory side effects?

Timolol is a beta-blocker often used in the treatment of glaucoma and hypertension. One of the known side effects of timolol, especially when used systemically or in eye drop form, is respiratory distress. This is particularly relevant in patients who have underlying conditions like asthma or chronic obstructive pulmonary disease (COPD), where the bronchoconstrictive properties of beta-blockers can exacerbate respiratory issues.

In contrast, cyclopentolate is primarily an anticholinergic medication used to dilate pupils and does not typically have respiratory side effects. Pilocarpine is a cholinergic agent that stimulates saliva and tear production and is also not associated with respiratory complications. Zinc sulphate is more commonly used as a dietary supplement or in treating zinc deficiency and generally has limited side effects related to the respiratory system. Thus, timolol stands out clearly due to its capacity to affect respiratory function in susceptible individuals.