A 24-year old female patient reports side effects after using Patanol for ocular allergies. Which symptom is most likely associated with olopatadine use?

Olopatadine, marketed under the name Patanol, is an antihistamine used to treat allergic conjunctivitis. While it is generally well-tolerated, some patients may experience side effects. Headaches are among the most commonly reported side effects of olopatadine. This can occur due to the way the medication interacts with histamine receptors, which may affect vascular responses in the brain and lead to tension-type headaches.

Other potential side effects of olopatadine may include mild transitory ocular irritation or dryness, but those are not as common. The presence of headaches as a side effect can often be attributed to the systemic absorption of the drug and its influence on central nervous system pathways.

Considering the other options, tachycardia is not a typical side effect of olopatadine; its mechanism primarily targets peripheral H1 receptors without significant systemic cardiovascular effects. Visual hallucinations are also unlikely, as they are not commonly associated with the typical antihistamine profile. Gastrointestinal discomfort is not specifically linked to olopatadine use and is more often seen with oral antihistamines rather than topical agents. Thus, headaches represent the most likely adverse effect for a patient using olopatadine for ocular allergies.