Over-the-counter ("first-generation") antihistamines often make you feel sleepy or tired. They may also affect your coordination, even when they do not make you drowsy. Because of this, you should not take them before you drive or operate machinery.
Another common side effect is a dry mouth. Taking them at bedtime may help with side effects.
Don't give antihistamines to your child unless you've checked with the doctor first.
Decongestants.Decongestants clear up a stuffy (congested) nose. They may be pills or liquids (oral), or a nasal gel, drop, or spray.
Possible problems with nasal sprays include irritation, burning or itching of nasal passages, and sneezing. Overuse can make your congestion worse (rebound congestion). So don't use the medicine longer than the label says. Examples of spray decongestants are oxymetazoline (such as Afrin, Dristan, or Zicam Extreme Congestion Relief) and phenylephrine (such as Neo-Synephrine).
Note: Decongestants may not be safe for young children or for people who have certain health problems. Before you use them, check the label. If you do use these medicines, always follow the directions about how much to use based on age and in some cases weight.
Steroid nasal sprays. These clear up a stuffy nose. They also help relieve red, itchy, watery eyes. An example is Nasacort.
Antihistamine/decongestant combinations. These combination pills work on most of the symptoms of allergies. Usually the decongestant decreases the drowsiness caused by the antihistamine. But some people feel nervous and sleepy at the same time ("tired and wired"). Examples of over-the-counter antihistamine/decongestant combinations include pseudoephedrine/chlorpheniramine maleate (such as Allerest) and pseudoephedrine/triprolidine (such as Actifed).
Decongestant eyedrops. These medicines reduce itching and watering of eyes. Don't use them for more than 3 days in a row. They can cause symptoms when you are not having allergy symptoms. This effect is similar to the rebound congestion of nasal spray decongestants. Examples of over-the-counter eyedrops include naphazoline (Clear Eyes) and tetrahydrozoline (such as Visine). (Saline-only eyedrops for dry eyes may feel good but do not reduce allergy symptoms.)