Some types of antihistamines produce slightly different side effects. For example, older forms of antihistamines are known to affect coordination and make it harder to concentrate. More recent versions no longer have this effect.

Side effects associated with antihistamines may diminish after several days of use. However, a person should immediately contact a physician upon experiencing any of the following serious side effects:

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

Less serious side effects of antihistamines include:

• Drowsiness (more common with over-the-counter antihistamines)
• Dry mouth or a bitter taste in mouth
• Nausea
• Restlessness or irritability
• Blurred vision
• Constipation

Children or elderly persons taking antihistamines may experience more intense side effects or other conditions, such as:

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