Antihistamines fight the production of histamines during nasal inflammation, which is often triggered by allergic rhinitis. The resulting itching, sneezing, irritated eyes and runny nose can be chronic, seasonal or triggered by certain plants, animals or chemicals. Antihistamines may give some symptomatic relief but don't cure the condition. Some people avoid using antihistamines due to side effects from the medication.
Drowsiness
Newer antihistamines may have less of a sedation effect and cause less drowsiness than older medications, but all antihistamines may cause drowsiness. The Mayo Clinic says antihistamines may magnify the effects of central nervous system depressants like alcohol and may cause extreme fatigue. Therefore, users are cautioned not to drive or operate machinery while using antihistamines.
Dry Mouth, Nausea
Treating a runny nose with an antihistamine may cause dry mouth and nausea. The Nasonex website states antihistamine medications are available in liquid, tablet or nasal mist form. It also cautions that although antihistamines may relieve itching and sneezing, it may cause dry mouth and nausea.
Difficulty Urinating
Men who suffer from benign prostatic hyperplasia (BPH) may have trouble urinating while using antihistamines. The Harvard Medical School states that urination problems are especially problematic if BPH sufferers are taking high doses of antihistamines.
Hypertension
Since antihistamines are often combined with decongestants like phenylephrine and pseudoephedrine, they may be dangerous for those with high blood pressure. The Mayo Clinic says decongestants narrow blood vessels, which can clear nasal congestion but may increase blood pressure at the same time.
