If you use over-the-counter (OTC) allergy medicines to help you get through pollen season or, life with your precious furball, you might find yourself wondering if it’s possible to get used to these. It can sometimes seem like tried-and-true remedies stop working as well as they once did. So, what’s really going on?

There are several reasons you might notice fewer benefits from your allergy medications, and it might take a visit with a healthcare professional to figure out what’s going on, says William Reisacher, MD, an otolaryngic allergist at Weill Cornell Medicine and NewYork-Presbyterian Hospital in New York City.

This includes medications such as:

• betamethasone (Nasacort)

• cetirizine (Zyrtec)

• fexofenadine (Allegra)

• fluticasone (Flonase)

• oxymetazoline (Afrin)

Here are some of the most likely culprits.

You’ve Developed a Medication Tolerance

Your body can get used to certain allergy medications over time. When that happens, a typical dose won’t bring you the relief it used to, and you’ll need more to produce the same effect.It doesn’t take long for this to happen, either. You can develop a tolerance to over-the-counter decongestant sprays, such as oxymetazoline (Afrin), in as little as three to five consecutive days of use, Dr. Reisacher says.Certain antihistamines, including cetirizine (Zyrtec), can also lose some effectiveness over time, especially when people take them for years.

You Have Rebound Congestion

Overusing decongestant nasal sprays, such as oxymetazoline (Afrin), can also lead to a type of nasal congestion due to inflammation in your nasal passages called rebound congestion (or rhinitis medicamentosa).These medications narrow blood vessels. But the blood vessels may become so narrow or so used to this effect that they may overcorrect and swell, causing symptoms, says Joseph T. Inglefield III, MD, an allergist at Hickory Allergy and Asthma Clinic and Cough Center in Hickory, North Carolina. This is less common today than with decongestant sprays that used to be available, but it may still occur for some.

You Have New or Worsening Allergies

Something about your allergies themselves may have changed. You could have exposure to new allergens, such as an air quality issue in your home or construction kicking up dust and debris in your neighborhood, Reisacher says.

Or, your symptoms might be worse due to an exceptionally problematic pollen season. You’re not imagining things: Climate change is causing plants to produce roughly 20 percent more pollen than 50 years ago, making seasonal allergy symptoms more severe and extending the allergy season. “The way your brain will interpret that is ‘The medications I've been using are no longer working,’” he says.

You’ve Gotten Older

Some people find their allergies worsen or return with age, or they develop new allergies as they get older.But aging may also change the way you experience your symptoms. “For example, as we age, seasonal allergy symptoms can shift from being allergic in nature to being atrophic (where the issue isn’t pollen but the underlying tissue is the nose itself changing and becoming weaker), producing similar symptoms that will not respond to medicines geared for allergies,” says Matthew Greenhawt, MD, a member of the Medical Scientific Council at the Asthma and Allergy Foundation of America (AAFA), board-certified in pediatrics and allergy and immunology in Denver. (Atrophic symptoms can also occur from overusing nasal decongestant sprays.)Older adults are also more likely to take other medications. Some of these, including aspirin and ibuprofen (Advil), may interfere with how well allergy medications perform, making you feel like your usual meds don’t work as well, Reisacher adds.

You’re Under a Lot of Stress

Although the underlying mechanism is not yet totally clear, research suggests that psychological stress may trigger or worsen nasal allergy symptoms.Stress might also change your perception of your symptoms. “When you’re under a lot of emotional or physical stress, you might perceive a symptom as being more significant in your life, whereas, if you're maybe on vacation in the Bahamas, and you get that stuffy nose, you're not even really caring about it anymore,” Reisacher says. Either way, your meds may not feel as effective because of stress.

You Don’t Just Have Allergies

Your symptoms might not be from allergies. You could have similar symptoms for some other reason, like a cold or the flu. Allergy medications work on allergies, not infections, so they won’t ease all your symptoms if you’re not addressing the underlying issue, Inglefield explains.Hormonal changes such as pregnancy, menstruation, and oral contraceptive use; ibuprofen, aspirin, and high blood pressure medications; and spicy foods and alcohol can all cause congestion that might make you think you’ve gotten used to your allergy meds.

You’re Not Taking Your Meds as Directed

If you’re not getting relief, double-check that you’re following instructions. “For example, a main reason for loss of effect is that people may not be taking the drugs as directed,” Dr. Greenhawt says.

Nasal steroids, such as fluticasone (Flonase) or betamethasone (Nasacort), should be taken every day, for example, but if you only use yours once or twice a week, you won’t notice much relief, he explains.Some estimates suggest most people with allergy symptoms don’t stick with their treatment plans: Anywhere from 35 to 70 percent don’t take their medication the way their doctor recommended or according to the product’s label.

If your current allergy medication regimen isn’t working, here are some strategies to try.

1. Change Your Antihistamine Dose

If you take an antihistamine like cetirizine daily but you’re no longer getting relief, increasing your dose may help. Under the guidance of a healthcare provider, you may be able to safely take more, he adds. That might mean taking a pill in the morning and in the evening instead of one daily, Inglefield says.

2. Try a Different Antihistamine

If cetirizine stops working, it’s possible you’d respond better to fexofenadine (Allegra), for example. These meds are in the same class but can work differently for different people. “Sometimes you have to try and see which one works best for you and gives you the best effect with the lowest amount of side effects,” Reisacher says. Or, your response might change over time. “One antihistamine works for a while, and then they feel like it's not working as well,” Inglefield says.

3. Use Medications as Directed

If you think you’ve been building tolerance to a decongestant nasal spray, it might feel natural to use it more often. But this can backfire, as in the cases of tolerance and rebound congestion above, and you may end up having more of the symptoms you’re hoping to avoid.

Reisacher recommends using decongestant nasal sprays only on your worst allergy days when you really need them, rather than using them regularly.On the other hand, if you take antihistamines, you should use them daily, Inglefield says.

4. Scale Back Carefully

If you’ve been using decongestant nasal sprays excessively, slowly scale back under the guidance of a healthcare provider. Otherwise, your symptoms might get worse.After decreasing your dose and frequency, you can likely go back to using the meds again as intended. “Now those medications will have utility when you actually need them,” Reisacher says. “They'll be much more potent for you when you use them properly.”

5. Get Allergy Testing

Make sure you know what you’re actually allergic to! This way you and your healthcare provider can determine the safest and most effective treatment options, Reisacher says.

This will likely involve skin prick testing to see how your body reacts to contact with certain allergens. Once you know what you’re allergic to, you can do your best to avoid those triggers as much as possible to limit your symptoms, Inglefield says.This information can also help you navigate treatment options. Your doctor can tell you if you’re a candidate for allergy shots, in addition to going over your most appropriate OTC options, he says.

• Allergy medications can stop working if you build up a tolerance to them, if your allergies change, if you’re not using them properly, or if you have an untreated infection that’s causing similar symptoms.

• To find relief from allergy symptoms, you may need to stop using a certain medication, try a different medication, or make sure you are taking the medications correctly (the right dose and the right time) as instructed on the packaging or by your doctor.

• Work with an allergist to help you identify what you’re truly allergic to and pinpoint the safest and most effective treatment options, including how to avoid triggers.