Olopatadine is a medication used to decrease the symptoms of allergic conjunctivitis and allergic rhinitis (hay fever). It is used as eye drops or as a nasal spray. The eye drops general result in an improvement within half an hour. Olopatadine is a medication used to decrease the symptoms of allergic conjunctivitis and allergic rhinitis (hay fever). It is used as eye drops or as a nasal spray. The eye drops general result in an improvement within half an hour. Common side effects include headache, sore throat, eye discomfort, change in taste. More significant side effects may include sleepiness. It is unclear if use during pregnancy or breastfeeding is safe. It is an antihistamine and mast cell stabilizer. Olopatadine was patented in 1986 and came into medical use in 1997. It is available as a generic medication. A 5 milliliter bottle of the eye drops in the United Kingdom costs the NHS less than £5 as of 2019. In the United States the wholesale cost of this amount is about US$12.50. In 2016 it was the 269th most prescribed medication in the United States with more than a million prescriptions. It is used to treat allergic conjunctivitis and hay fever. It is used as eye drops and as a nasal spray. Some known side effects include headache (7% of occurrence), eye burning and/or stinging (5%), blurred vision, dry eyes, foreign body sensation, hyperemia, keratitis, eyelid edema, pruritus, asthenia, sore throat (pharyngitis), rhinitis, sinusitis, taste perversion, and vomiting. Olopatadine acts as a selective antagonist of the histamine H1 receptor, thus stabilizing mast cells and inhibiting histamine release. Olopatadine was patented in 1986 by Kyowa Hakko Kogyo and came into medical use in 1997. Brand names include Pazeo, Pataday, Patanol S, Patanol, Opatanol, Olopat, Patanase. It is also available as an oral tablet in Japan under the tradename Allelock, manufactured by Kyowa Hakko Kogyo.