Nasal inflammation causes your immune system to release histamines and leukotrienes, with accompanying mucous and the irritation of a "runny nose." Antihistamines combat the production of histamines, which is often caused by allergic rhinitis. While antihistamines give symptomatic relief, they can't really fix the underlying problem. And besides only giving symptomatic relief, antihistamines come with side effects.
Sedation
Antihistamines cause a sedating sleepiness as a side effect that is reflected in packaged warnings not to drive an automobile or operate machinery when using the drug. Mayo Clinic staff warn that antihistamines are one of the central nervous system depressants which can make you very tired and may have an exaggerated effect when combined with other depressants like alcohol.
Stomach Upset and Dry Mouth
Antihistamines tend to give some people an upset stomach and dry mouth. The Nasonex website lists the various possible antihistamine delivery vehicles as tablet, liquid or mist forms. But it also warns that along with drying up nasal congestion, your mouth may feel dry and you may become nauseous from using antihistamines.
Bladder Difficulties
Antihistamines may make urination difficult for men with benign prostatic hyperplasia (BPH). The Harvard Medical School website warns that difficulty urinating is especially problematic when you combine the enlarged prostate symptoms of BPH with use of antihistamines.
High Blood Pressure
Those with high blood pressure problems need to be especially careful taking antihistamines combined with decongestants like phenylephrine, and pseudoephedrine. Mayo Clinic staff say decongestants like these constrict blood vessels and increase blood pressure. If you already have a problem with high blood pressure, your hypertension could be elevated to a dangerous level when spurred on by a decongestant.
