This information is for people who have hay fever. It tells you about antihistamines and pseudoephedrine, a treatment used for hay fever. It is based on the best and most up-to-date research.
Do they work?
Yes. Taking both an antihistamine tablet and a decongestant medicine called pseudoephedrine can help with the symptoms of hay fever. This combination may work well for you if your hay fever symptoms include feeling blocked up and having difficulty breathing through your nose.
But you may have more side effects if you use this combined treatment than if you take antihistamine tablets on their own. The most common side effects are headaches and problems sleeping.
What are they?
Antihistamines help with many of the symptoms of hay fever by calming your body's allergic reaction to pollen. They do this by blocking the action of a chemical in your body called histamine.   Some common brand names are Telfast, Zirtek, and Clarityn. To learn more, see Antihistamine tablets.
Pseudoephedrine is a decongestant. This means it works to clear your blocked nose and help you breathe more easily.  One brand name is Non Drowsy Sudafed Decongestant Tablets. To learn more, see Decongestants.
In the UK, decongestants such as pseudoephedrine are not often combined with antihistamines in one tablet as a treatment for hay fever. There is one product for hay fever that contains pseudoephedrine and an antihistamine called acrivastine. Its brand name is Benadryl Plus, and you can buy it from a pharmacy. It's unlikely to make you feel sleepy.
How can they help?
Taking an antihistamine and medicine containing pseudoephedrine can make you feel better by relieving the symptoms of hay fever. It doesn't seem to matter what type of antihistamine you take. Several studies have shown that if you take this treatment:        
You may sneeze less often
Your nose will feel less itchy
Your nose will be less runny
Your eyes will be less itchy
Your nose will be less blocked and you will be able to breathe more easily. Most antihistamines taken alone do not help with a blocked nose.
How do they work?
If you have hay fever, you're probably allergic to pollen. When you breathe in pollen, your immune system, which normally helps to protect you from illness, reacts as if pollen is something that might harm you.
Your immune system makes your body release powerful chemicals to try to protect you. One of these is called histamine. Histamine causes the symptoms of hay fever, such as sneezing, a runny or blocked nose, and red, itchy, or watery eyes.
Antihistamines stop the histamine working. So your symptoms should calm down and you should feel better. 
But another symptom of hay fever is a blocked nose. This happens because your body's reaction to pollen makes the inside of your nose swell and causes your nose to make lots of mucus. Pseudoephedrine helps this swelling go down so that mucus can drain from your nose.  And this makes it easier for you to breathe.