The allergy and cold aisle contains dozens of products, but most of them contain some combination of just two active ingredient types: antihistamines and decongestants. Understanding the difference between them makes choosing much simpler, and it ensures you are treating the symptom that is actually bothering you.
What an Antihistamine Does
Histamines are chemicals released by your immune system when it detects an allergen, pollen, pet dander, dust mites, mold. Those histamines bind to receptors throughout the body, triggering allergy symptoms: sneezing, runny nose, itchy eyes, and skin reactions.
An antihistamine blocks those histamine receptors, it does not prevent allergen exposure, but it stops histamine from causing its effects. Because the mechanism is about histamine, antihistamines work best for symptoms that are driven by an allergic response.
Best for:
- Sneezing
- Runny nose (watery, clear discharge)
- Itchy, watery eyes
- Itchy skin, hives
- Allergic rhinitis (hay fever)
- Post nasal drip from allergies
Common OTC antihistamines: loratadine (Claritin), cetirizine (Zyrtec), fexofenadine (Allegra), diphenhydramine (Benadryl)
What a Decongestant Does
Congestion, that stuffy, blocked feeling in your sinuses, is caused by swollen blood vessels and inflamed tissue in the nasal passages. This is a vascular problem, not a histamine problem. Antihistamines do not do much for this, particularly when the swelling is from a cold or sinusitis.
A decongestant works by narrowing (constricting) blood vessels in the nasal lining. This reduces swelling and fluid accumulation, improving airflow and relieving the plugged up feeling.
Best for:
- Nasal and sinus congestion (stuffiness)
- Sinus pressure
- Ear pressure secondary to congestion
- Congestion from colds (not driven by histamine)
Common OTC decongestants:
- Pseudoephedrine (Sudafed PE, not Sudafed original; note: the original Sudafed, now kept behind the counter under federal law, contains pseudoephedrine)
- Phenylephrine (available on the shelf as Sudafed PE), note that a 2023 FDA advisory committee found oral phenylephrine ineffective at the approved OTC dose; consult a pharmacist
- Oxymetazoline (Afrin), nasal spray; very effective but should not be used more than 3 consecutive days due to rebound congestion risk
When to Use Both Together
Many symptoms combine a histamine component and a vascular/inflammatory component, and many OTC combination products are designed for this. The "-D" suffix on many allergy medications indicates they contain a decongestant:
- Claritin D = loratadine + pseudoephedrine
- Zyrtec D = cetirizine + pseudoephedrine
- Allegra D = fexofenadine + pseudoephedrine
These combination products provide broad symptom relief when you have both runny/sneezy symptoms (antihistamine needed) and congestion/sinus pressure (decongestant needed). However, because they contain pseudoephedrine, they are kept behind the pharmacy counter.
Use a combination product when you have:
- Runny nose AND stuffy nose at the same time
- Seasonal allergy symptoms with significant sinus congestion
- Cold symptoms with both watery discharge and blockage
Side Effects to Know Before Choosing
Antihistamines: The main side effect is sedation, which varies by generation. First generation (Benadryl) is significantly sedating. Second generation (Claritin, Zyrtec, Allegra) is much less so, though cetirizine can cause mild drowsiness in some people.
Decongestants: Pseudoephedrine can raise blood pressure and heart rate, cause insomnia, and cause urinary retention. This matters for:
- People with hypertension, ask your prescriber before using
- Men with enlarged prostate (BPH), decongestants can worsen urinary symptoms
- People with heart arrhythmias or hyperthyroidism
- Pregnant women, discuss with provider before use
Nasal oxymetazoline (Afrin) avoids the systemic effects but still causes rebound congestion when used beyond three days.
Decision Framework
When to Talk to a Pharmacist
This is one of the clearest reasons to ask a pharmacist before grabbing something off the shelf:
- You have high blood pressure and want to use a decongestant
- You have an enlarged prostate and are considering a decongestant
- You are pregnant or breastfeeding
- You are choosing for a child
- You are not sure whether your symptoms are allergy driven or from a cold (it changes the recommendation)
- You want pseudoephedrine containing products and have questions about the purchase process
You can browse allergy and cold essentials at Fairview Pharmacy, with pharmacists available to help narrow down the right product.
This article is for general information only and is not a substitute for personalized medical advice. Before starting or changing any medication, including over the counter products and supplements, talk with your pharmacist or physician about your specific situation.