Side effects tend to vary according to the type of antihistamine used. For example, older types of first generation antihistamines are known to affect coordination and make it harder to concentrate. More recent versions no longer have this effect. The side effects associated with antihistamines sometimes diminish after several days of use.

A person should immediately contact a physician if they experience any of the following side effects of antihistamines:

• Low blood pressure
• Dizziness
• Inability to urinate
• Labored breathing
• Increased heartbeat
• Vomiting

Less serious side effects of antihistamines include:

• Drowsiness
• Dry mouth or a bitter taste in mouth
• Nausea
• Restlessness or irritability
• Blurred vision
• Constipation

Children or elderly people taking antihistamines may experience more intense or additional side effects such as:

• Seizures
• Confusion and nervousness
• Dryness of the nose, mouth or throat
• Difficult or painful urination
• Nightmares