Actually, they may not be. Depending on the type of anthistamine, you could end up suffering from elevated blood pressure, low blood pressure and even addictions.
Antihistamines that contain phenylpropanolamine are not only dangerous, they�re addictive. There are also steroidal and non-steroidal antihistamines that can have an impact on your menstrual cycle as well as affecting the fine membrane tissue in your nasal passage.
Please, I encourage you to have a frank discussion with your GP. You may find she prescribes alternatives that provide you with greater relief and are safer as well.
I wish you every success
Fr Bill