People with allergies use decongestants such as pseudoephedrine (Sudafed, Actifed, others) or phenylephrine (Sudafed PE) to constrict the small blood vessels in the nose.
These decongestants do offer relief, but don’t stop at constricting only nasal blood vessels. They constrict all blood vessels throughout the body, thereby making the heart work harder to pump blood, thereby increasing blood pressure.
If you’re taking an over-the-counter decongestant, choose one that doesn’t contain pseudoephedrine or phenylephrine, such as cetirizine (Zyrtec), chlorpheniramine, (Chlor-Trimeton), loratadine (Claritin) and dephenhydramine (Benadryl). All can help relieve congestion and are safer for the heart.
Another option is to use a nasal spray, which acts directly on nasal blood vessels and has less impact on other blood vessels in the body.
If your allergies promote nasal congestion at night, they might be interfering with your breathing while you sleep. This interference can lead to or worsen the type of fighting for breath known as sleep apnea, which in itself can cause high blood pressure.
Should your sleeping partner tell you that you snore and sound as though you’re choking or gasping for breath at times, ask your doctor to check you for sleep apnea.
from Harvard School of Public Health (HSPH) Healthbeat, May 4, 2012