Complete Guide to Allergy Medications
A practical overview of allergy medications — antihistamines, decongestants, corticosteroids, and immunotherapy — covering how they work, OTC versus prescription options, and which to use for different allergy symptoms.
How Allergic Reactions Work
An allergic reaction begins when the immune system incorrectly identifies a harmless substance (pollen, dust mites, pet dander, certain foods) as a threat. On first exposure, the body produces IgE antibodies that attach to mast cells — immune cells concentrated in tissues that contact the outside world (nasal lining, airways, skin, gut).
On subsequent exposures, the allergen binds to those IgE antibodies on mast cells, triggering rapid release of histamine and other chemicals. Histamine binds to H1 receptors throughout the body, causing:
- Nasal congestion and runny nose
- Itchy, watery eyes
- Sneezing
- Skin hives and itching
- Bronchoconstriction in asthma
Allergy medications target different points in this process.
First-Generation Antihistamines
First-generation antihistamines (diphenhydramine/Benadryl, chlorpheniramine, hydroxyzine, clemastine) are antagonists at histamine H1 receptors — they block histamine from binding and causing symptoms. They work quickly (within 30–60 minutes) and are highly effective at relieving acute allergic symptoms.
The key drawback: first-generation antihistamines cross the blood-brain barrier, causing sedation and cognitive impairment. They also block muscarinic acetylcholine receptors, causing dry mouth, urinary retention, and constipation. These effects can be significant in older adults.
They are appropriate for: - Acute allergic reactions requiring fast relief - Short-term insomnia (marketed as sleep aids like ZzzQuil, Unisom) - Itching from hives or eczema when sedation is acceptable
Second-Generation Antihistamines
Second-generation antihistamines (loratadine/Claritin, cetirizine/Zyrtec, fexofenadine/Allegra, levocetirizine/Xyzal, desloratadine/Clarinex) were engineered to be more selective and to poorly cross the blood-brain barrier — dramatically reducing sedation.
Key differences: - Fexofenadine: Least sedating of the class; does not cause drowsiness even in high doses - Cetirizine and levocetirizine: Slightly more sedating than fexofenadine but still far better than first-generation agents; fast onset - Loratadine: Minimal sedation, moderate speed of action; now widely available as a generic
Second-generation antihistamines are ideal for daily allergy management. They are taken once daily (or twice for some formulations) and are almost universally available over the counter.
Nasal Corticosteroids
Intranasal corticosteroids (fluticasone/Flonase, budesonide/Rhinocort, triamcinolone/Nasacort, mometasone/Nasonex) are the most effective medications for allergic rhinitis (hay fever). They reduce inflammation in the nasal lining, decreasing congestion, runny nose, sneezing, and post-nasal drip.
Key facts: - Need to be used daily — consistent use produces better results than "as needed" - Full effect takes 1–2 weeks of regular use to develop - Minimal systemic absorption — local side effects (nasal dryness, occasional nosebleeds) are the main concern - Several are available OTC; others require a prescription
For most people with moderate-to-severe seasonal or perennial allergic rhinitis, a nasal steroid plus an oral antihistamine covers symptoms better than either alone.
Decongestants
Decongestants (pseudoephedrine, phenylephrine, oxymetazoline/Afrin) reduce nasal congestion by narrowing blood vessels in the nasal lining.
- Pseudoephedrine (Sudafed): the most effective oral decongestant; available behind the pharmacy counter due to its potential use in methamphetamine production
- Phenylephrine: sold on open shelves but clinical trials have questioned its effectiveness at standard OTC doses
- Oxymetazoline nasal spray: highly effective but causes rebound congestion (rhinitis medicamentosa) if used more than 3 consecutive days
Oral decongestants can raise blood pressure and heart rate — use with caution in people with hypertension, heart disease, thyroid disease, or glaucoma.
Nasal Antihistamines and Combinations
Nasal antihistamine sprays (azelastine/Astelin, olopatadine/Patanase) work directly in the nasal passages, with faster onset than oral antihistamines (15 minutes). They may cause some local drowsiness.
Dymista combines azelastine and fluticasone in one nasal spray — a highly effective dual-action option for moderate-to-severe allergic rhinitis.
Mast Cell Stabilizers
Cromolyn sodium (NasalCrom) prevents mast cells from releasing histamine when exposed to an allergen. It is most effective when started 1–2 weeks before allergy season begins — it is preventive, not reliever-based. It is very safe (minimal absorption) but requires dosing 3–4 times daily, which limits adherence.
Eye drop forms (cromolyn, nedocromil) are effective for allergic conjunctivitis and are well tolerated.
Immunotherapy: Allergy Shots and Tablets
Allergen immunotherapy is the only treatment that addresses the underlying immune cause of allergies, not just symptoms. It gradually desensitizes the immune system by repeated exposure to increasing amounts of the allergen.
- SubcutaneousSubcutaneous
A route of drug administration where medication is injected into the layer of fat and connective tissue just beneath the skin. Absorption is slower than IV or IM but is suitable for self-administratio
immunotherapy (SCIT): Injections given weekly for several months, then monthly for 3–5 years. Produces lasting toleranceToleranceA decrease in a drug's effect over time with repeated administration, requiring higher doses to achieve the same response. Tolerance develops through receptor downregulation, enzyme induction, or othe
. - SublingualSublingual
A route of drug administration where the medication is placed under the tongue, allowing absorption through the highly vascular sublingual mucosa directly into the bloodstream. This bypasses first-pas
immunotherapy (SLIT): Dissolving tablets or drops placed under the tongue daily at home. Approved tablets in the US include those for grass, ragweed, and dust mite allergies.
Immunotherapy is considered when: - Symptoms are not adequately controlled by medications - Medications cause intolerable side effects - Long-term reduction in allergy severity is desired - The patient is willing to commit to multi-year treatment
OTC Availability and Choosing Wisely
Most allergy medications are now available over the counter. General guidance:
| Symptom | Best OTC Option |
|---|---|
| Runny nose, sneezing, itchy eyes | Second-generation antihistamine |
| Nasal congestion (primary) | Nasal corticosteroid or pseudoephedrine |
| All-around seasonal allergies | Nasal corticosteroid + antihistamine |
| Acute hives or itch | First-generation antihistamine (short-term) |
If symptoms are well controlled on an OTC regimen, a doctor visit for prescription drugs is not necessary. Seek medical evaluation for severe reactions, asthma, or symptoms unresponsive to OTC treatment.
Key Takeaways
- Antihistamines block histamine H1 receptors (antagonists) — second-generation options are preferred for daily use due to minimal sedation.
- Nasal corticosteroids are the most effective daily treatment for allergic rhinitis but take 1–2 weeks to reach full effect.
- Decongestants reduce congestion but carry cardiovascular risks and potential for rebound congestion (nasal sprays).
- Mast cell stabilizers are preventive and work best when started before allergen exposure.
- Allergen immunotherapy is the only disease-modifying treatment, providing lasting relief after a multi-year course.
- Most allergy medications are now available over the counter.