Antihistamines are not known to cause nausea. However, it is possible that you are intolerant to this medication, which can lead to nausea. It is possible that the cause is something else. If you are taking medications on a regular basis, I recommend that you talk to your pharmacist to review your record and possible interactions with this medication. If the nausea persists tomorrow, I suggest that you stop the antihistamine and ask your pharmacist for Nasacort, Nasonex or Flonase. These are intranasal spray corticosteroids. It is a local treatment and therefore very unlikely to cause nausea. Furthermore, it is one of the best treatments for seasonal allergies. Please check all this with your pharmacist. Sincerely, Mac-Pherson Anacréon Pharmacist