During an allergic reaction, tissues and blood vessel tend to swell up and cause nasal congestion and redness in the eyes. Decongestants are usually prescribed to people to reduce the symptoms of allergies like congestion and redness. They come in the form of pills, nasal sprays, eye drop and liquid.

When tissues in the nose become inflamed, it causes excess mucus production and nasal congestion. Decongestants work by narrowing the blood vessels in the tissues of the nose. This reduces the blood flow to the tissues and causes the swollen tissues to shrink. When inflammation reduces, mucus production and congestion also decrease.
Allergic reactions can cause redness, burning and itching of the eyes. Decongestants constrict the blood vessels in the eyes. This reduces the blood flow through these vessels and thus reduces redness.

Eye and nasal decongestants cannot be used for prolonged periods of time because they can make symptoms worse if used for too long.

There are both over the counter and prescription decongestants that work to reduce the symptoms of an allergy.

It is safe for healthy adults to use decongestants once in a while, but it is not recommended for use in people who have high blood pressure or glaucoma.