Antihistamines are generally not known to cause nausea. However, it is possible that you are intolerant to this medication, which can cause nausea. It is also possible that the cause is something else. If you are taking medication on a regular basis, I recommend that you contact your pharmacist to review your file and any potential interactions with the medication you purchased at Costco. If the nausea persists until tomorrow, I suggest that you stop taking the antihistamine and ask your pharmacist about Nasacort, Nasonex, or Flonase. These are cortisone intranasal sprays, and they have a low chance of causing nausea. Additionally, they are some of the best treatments for seasonal allergies. Please check with your pharmacist for more information. Sincerely, Mac-Pherson Anacreon Pharmacist