Wayne Rawlins, MD, MBA Allergies can pop up throughout the year. You may be tempted to grab the closest antihistamine medicine to fend off your itchy eyes, sneezing, or a runny nose. Before you do, make sure the medicine won't do more harm than good. Subscribe There are different types of antihistamines, and some are better than others depending on your age, health, and other medications you take. Older formulas work, but beware of side effects and bad interactions Doctors often refer to the first type of antihistamines that were on the market as "first-generation." They are great at treating allergy symptoms. Unfortunately, in some people, they can result in side effects like anxiety, confusion, feeling sleepy, blurred vision, reduced mental alertness, urinary retention and constipation. These effects are more common for anyone taking certain antidepressant medications.