They're more useful if you use them before you're exposed to allergens. You can buy some antihistamines without a prescription. These relieve symptoms just as well as prescription medicines do, but they tend to cause drowsiness and dry mouth. This often improves after 1 to 2 weeks of regular use. Antihistamines that require a prescription from your doctor are less likely to cause tiredness or dry mouth.
Decongestants help temporarily relieve the stuffy nose of allergies.
They come as pills, nose sprays and nose drops. They are best used only for a short time. Nose sprays and drops shouldn't be used for more than 3 days because you can become dependent on them. This causes you to feel even more stopped-up when you try to quit using them. You can buy decongestants without a doctor's prescription.