How Long Do Antihistamines Take to Work?

Antihistamines are among the most commonly used medicines in the UK. Millions of people reach for them each year to manage symptoms of hay fever, hives, pet allergies, and other allergic conditions. One of the most frequent questions is how long they actually take to start working — and whether the type you are taking makes a difference.

This guide provides general information about how antihistamines work, how long different types typically take to provide relief, and what to consider if your current antihistamine does not seem to be helping. It is intended as patient information only — not as medical advice, diagnosis, or a recommendation to take any specific medicine. If you need personalised guidance, a pharmacist is a good first point of contact.

What Do Antihistamines Actually Do?

When your body encounters something it identifies as a threat — such as pollen, dust mites, or animal dander — your immune system may release a chemical called histamine. Histamine is involved in many allergic symptoms, including sneezing, itching, watery eyes, and skin rashes.

Antihistamines work by blocking histamine receptors (specifically the H1 receptor), which reduces the effect that histamine has on your body. They do not stop your immune system from producing histamine, but they prevent histamine from triggering the symptoms you experience during an allergic reaction.

This is why antihistamines tend to be most effective for symptoms that are directly driven by histamine — such as itching, sneezing, and a runny nose. A blocked or congested nose, which is caused more by inflammation and swelling, may respond less well to antihistamines alone. If nasal congestion is your main concern, a corticosteroid nasal spray may be more appropriate — a pharmacist can advise.

How Long Different Antihistamines Take to Work

The table below provides a general comparison of commonly available antihistamines in the UK. These are approximate timings based on general pharmacological information. Individual experience may vary, and you should always follow the patient information leaflet that comes with your medicine.

These timings are general guides only. How quickly any antihistamine works for you depends on individual factors including your metabolism, the severity of your symptoms, and whether you have taken the medicine with food.

Drowsy vs Non-Drowsy Antihistamines: What Is the Difference?

Antihistamines are broadly divided into two categories: first-generation (often called drowsy or sedating) and second-generation (often called non-drowsy). The main difference relates to how the medicine crosses the blood-brain barrier.

First-Generation (Drowsy) Antihistamines

First-generation antihistamines — such as chlorphenamine and diphenhydramine — were developed earlier and can cross the blood-brain barrier more readily. This means they are more likely to cause drowsiness and may affect concentration, reaction time, and the ability to drive safely.

These antihistamines tend to act relatively quickly (often within 30 minutes) but wear off faster, typically lasting four to six hours. They may be taken multiple times per day.

Second-Generation (Non-Drowsy) Antihistamines

Second-generation antihistamines — such as cetirizine, loratadine, and fexofenadine — are designed to be less likely to cross the blood-brain barrier, which means they generally cause less drowsiness. They are the most commonly recommended type for daytime use in the UK.

These antihistamines typically last longer (around 24 hours), making them convenient for once-daily dosing. However, "non-drowsy" does not mean drowsiness is impossible — some individuals may still experience mild tiredness, particularly with cetirizine.

If you are driving or operating machinery, non-drowsy antihistamines are generally the more appropriate choice. If you notice drowsiness after taking any antihistamine, avoid driving until you understand how it affects you. A pharmacist can help you choose the most suitable option for your situation.

How Long Do Antihistamines Take to Work for Common Conditions?

The speed and degree of relief you experience can depend on the condition being treated. Below is a general overview.

Hay Fever

For hay fever symptoms such as sneezing, itching, and a runny nose, antihistamines often provide some improvement within 30 to 60 minutes. Nasal congestion may respond less well to antihistamines alone and may benefit from a corticosteroid nasal spray. Taking antihistamines regularly throughout the pollen season — rather than only when symptoms appear — tends to provide better overall control for many people.

Hives (Urticaria)

Antihistamines are a common first-line approach for hives and typically provide noticeable improvement within the same day. However, for persistent or chronic hives, consistent daily use over several days may be needed before the full benefit becomes apparent. If hives are not settling with over-the-counter antihistamines used as directed, seek further advice from a pharmacist or healthcare professional.

Pet Allergies

If you experience symptoms around cats, dogs, or other animals, antihistamines can help reduce itching, sneezing, and watery eyes. However, if you are regularly exposed to a pet allergen, the antihistamine is working against a continuing source of histamine release, which may limit its effectiveness. Reducing exposure where possible — alongside taking an antihistamine — often gives better results.

Dust Mite Allergy

Symptoms triggered by dust mites — such as morning sneezing, a runny nose, or itchy eyes — may respond to antihistamines within an hour. However, because dust mite exposure is often continuous (particularly in bedding and soft furnishings), environmental measures such as allergen-proof covers and regular washing of bedding at 60°C may help antihistamines work more effectively.

If You Rely on Antihistamines Frequently

Many people take antihistamines occasionally during peak pollen season or when they encounter a known trigger. However, if you find yourself taking antihistamines most days — or if you are not sure exactly what is causing your symptoms — it may be worth considering whether understanding your specific allergen triggers could support more targeted management.

A specific IgE blood test measures whether your immune system has produced antibodies to particular allergens. This can help build a clearer picture of what you may be reacting to — whether that is grass pollen, tree pollen, dust mites, animal dander, or certain foods.

It is important to understand the limitations of IgE testing. A positive result indicates sensitisation — meaning your immune system has responded to a particular allergen — but sensitisation does not always equate to clinical allergy. Results are most meaningful when interpreted alongside your symptom history and exposure patterns, ideally with input from a qualified healthcare professional.

Importantly, allergy blood tests are not typically affected by antihistamine use. Unlike skin prick tests, which require you to stop antihistamines beforehand, blood-based IgE testing can generally be carried out while you continue taking your usual medicine.

Why Your Antihistamine May Not Be Working

If you have been taking an antihistamine consistently and your symptoms are not improving, there are several factors worth considering before concluding that the medicine has failed.

• Wrong type for the symptom: Antihistamines are most effective for histamine-driven symptoms like itching, sneezing, and a runny nose. Nasal congestion is driven more by inflammation and may require a different approach such as a corticosteroid nasal spray.
• Not enough time: While many antihistamines begin working within an hour, some conditions — particularly chronic hives — may need several days of consistent use before improvement becomes clear.
• Ongoing allergen exposure: If you are continuously exposed to the trigger (for example, pet dander in the home or dust mites in bedding), the antihistamine may be unable to fully control symptoms. Reducing exposure where possible may help.
• Individual variation: Differences in metabolism, body weight, and how your body processes the medicine can influence effectiveness. What works well for one person may not work as well for another.
• Tolerance: Some people report that an antihistamine seems to become less effective over time. While this is not well established for second-generation antihistamines, switching to a different type — under the guidance of a pharmacist — is sometimes suggested.

If symptoms persist despite appropriate self-care, speak to a pharmacist. They can review what you are taking, check for potential interactions, and suggest alternative options.

Safety and When to Seek Urgent Help

While antihistamines are generally well tolerated, it is important to recognise situations that require immediate medical attention. The following symptoms may indicate a severe allergic reaction (anaphylaxis) and should be treated as a medical emergency:

• Difficulty breathing, wheezing, or feeling unable to get enough air
• Swelling of the lips, tongue, or throat
• Collapse, feeling faint, or losing consciousness
• Rapidly worsening symptoms despite taking an antihistamine

If you experience these symptoms, seek emergency medical care immediately by calling 999 or attending A&E. If you have an adrenaline auto-injector, use it as directed and call for help. Do not rely on antihistamines alone in an emergency situation.

For non-urgent concerns — such as drowsiness, suitability during pregnancy or breastfeeding, use in children, or potential interactions with other medicines — speak to a pharmacist for personalised guidance.

Frequently Asked Questions

References

1. NHS — "Antihistamines" (NHS patient information page)
2. NHS — "Hay fever" (NHS Health A to Z)
3. NHS — "Hives (urticaria)" (NHS patient information)
4. NICE Clinical Knowledge Summaries — "Allergic rhinitis: management"
5. British National Formulary (BNF) — antihistamine monographs (general pharmacological reference)
6. Allergy UK — "Treatments for allergy" (patient information)
7. DVLA — guidance on medicines and driving fitness