How Long Do Antihistamines Take to Work?
Whether you are managing hay fever symptoms during London's pollen season or dealing with persistent hives, one of the most common questions people have about antihistamines is how quickly they start working. The answer depends on several factors, including the type of antihistamine, the symptoms being treated, and individual variation.
This article provides general information about antihistamine onset times, how long different types tend to last, and what to do if your 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 are unsure which antihistamine is right for you, a pharmacist is a good first point of contact.
In Most Cases, How Long Do Antihistamines Take to Work?
Most oral antihistamines begin to take effect within 30 to 60 minutes of being taken. Peak effectiveness is often reached within one to two hours. The duration of action varies — many modern non-drowsy antihistamines provide relief for approximately 12 to 24 hours. However, how quickly and how well an antihistamine works can vary from person to person and may depend on the type and severity of symptoms.
What Does "Start Working" Actually Mean?
When people ask how long for an antihistamine to work, they often mean different things. It is helpful to distinguish between three stages of antihistamine action: first noticeable relief, peak effect, and duration.
First noticeable relief is the point at which you begin to feel some improvement in symptoms. For many oral antihistamines, this may happen within 30 to 60 minutes. You might notice that itching eases, sneezing reduces, or watery eyes begin to settle. However, this initial improvement does not necessarily mean the medicine has reached its full effect.
Peak effect refers to the point at which the antihistamine is working at its maximum capacity. This typically occurs one to two hours after taking the medicine. At this stage, you are likely to experience the fullest level of symptom relief that the antihistamine can provide.
Duration of action describes how long the effect lasts before the medicine begins to wear off. For modern non-drowsy antihistamines, this is often between 12 and 24 hours, which is why many are taken once daily.
It is also worth noting that antihistamines tend to be most effective for symptoms driven by histamine release — such as itching, sneezing, and a runny nose. A blocked or congested nose may respond less well to antihistamines alone, as congestion is driven more by inflammation and swelling than by histamine activity directly. If nasal blockage is your main concern, a pharmacist can advise on additional options.
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.
| Medicine Type | Common Use | Typical Onset | How Long It Lasts | Drowsiness Risk |
|---|---|---|---|---|
| Second-generation (non-drowsy) | ||||
| Cetirizine | Hay fever, hives, skin itching | Many people notice improvement within 30–60 minutes | Typically around 24 hours | Low, though some individuals may experience mild drowsiness |
| Loratadine | Hay fever, hives, skin itching | Typically begins to work within 1–3 hours | Typically around 24 hours | Generally low |
| Fexofenadine | Hay fever, hives | Many people notice improvement within 1–2 hours | Typically around 24 hours | Generally low |
| First-generation (more sedating) | ||||
| Chlorphenamine | Hay fever, hives, itching, insect bites | Typically begins to work within 30 minutes | Approximately 4–6 hours | May cause drowsiness in many individuals; can affect driving |
| Diphenhydramine | Itching, short-term sleep support | Typically begins to work within 30–60 minutes | Approximately 4–6 hours | Commonly causes drowsiness; not suitable for daytime driving |
If you are driving or operating machinery, non-drowsy antihistamines are generally the more appropriate choice. However, individual responses to any antihistamine can vary. If you notice drowsiness after taking a medicine labelled as non-drowsy, avoid driving until you understand how it affects you. A pharmacist can help you choose the most suitable option for your circumstances.
How Long Do Antihistamines Take to Work for Different Conditions?
The speed and degree of relief you experience from an antihistamine can also depend on the condition being treated. Below is a general overview of how antihistamines tend to perform for some of the most common allergy-related symptoms.
Hay Fever (Allergic Rhinitis)
For hay fever, antihistamines often help with sneezing, itching, and a runny nose relatively quickly — many people notice some improvement within an hour or so. However, nasal congestion and blockage may respond less well to antihistamines alone. If congestion is your main symptom, additional options such as a corticosteroid nasal spray may be worth discussing with a pharmacist. How long antihistamines take to work for hay fever depends partly on severity and whether you are taking them consistently or only when symptoms appear.
Hives (Urticaria)
Antihistamines are commonly used for hives and often 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, it is sensible to seek further advice from a pharmacist or healthcare professional.
Skin Itching
Relief from general skin itching is often noticeable within a few hours of taking an antihistamine. For some people, this may be one of the faster-responding symptoms. Antihistamine creams are also available, but oral antihistamines are generally preferred for widespread itching. If itching is persistent or accompanied by a rash, seeking professional assessment is advisable.
Eye Symptoms
Itchy, watery eyes caused by allergy (allergic conjunctivitis) can often be managed with oral antihistamines. Antihistamine eye drops are also available and may act faster because they deliver the medicine directly to the affected area. A pharmacist can advise on whether eye drops would be a suitable addition to your current approach.
Why Antihistamines May Work Faster — or Slower
If you have noticed that your antihistamine seems to work quickly on some occasions but not others, several factors may explain this variation. Understanding these can help set realistic expectations.
The severity of your symptoms at the time you take the medicine plays a role. If symptoms are mild and caught early, an antihistamine may bring noticeable relief more quickly. If symptoms have been building for some time — for example, after prolonged outdoor pollen exposure during a London summer — it may take longer for the medicine to bring things under control.
Ongoing allergen exposure can also affect how quickly antihistamines work. If you are still being exposed to the trigger (for example, spending time in a pollen-heavy environment or around a pet), the antihistamine is working against a continuing source of histamine release, which may limit its effectiveness.
Consistency of use matters for some conditions. For persistent symptoms such as seasonal hay fever, taking an antihistamine regularly and consistently — rather than only when symptoms flare — often provides better overall control.
Whether the medicine matches the symptom type is also relevant. As noted above, antihistamines are most effective for histamine-driven symptoms like itching and sneezing. They are less effective for congestion, which may require a different approach.
Finally, individual biological differences — including metabolism, body weight, and how your body processes the medicine — can influence how quickly and how well an antihistamine works for you. This is normal variation and does not necessarily mean the medicine is ineffective.
What If Your Antihistamine Is Not Working?
If you have been taking an antihistamine consistently and your symptoms are not improving, there are several practical steps worth considering before concluding that the medicine has failed.
First, confirm that you are using the right type of antihistamine for your symptoms. Non-drowsy antihistamines are generally well suited to daytime use for hay fever and hives, but nasal congestion may need a different approach. A pharmacist can help assess whether the type of antihistamine you are using is a good match for your main symptoms.
Second, allow 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. Stopping too early can give the impression that the medicine is not working when it simply has not had enough time.
Third, consider whether ongoing environmental triggers may be undermining the effect. If you are regularly exposed to allergens — such as dust mites in bedding or pet dander in the home — reducing exposure where possible may help your antihistamine work more effectively.
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 or additional treatments that may be more suitable for your situation.
When Allergy Testing May Be Helpful
If you find yourself relying on antihistamines frequently — particularly if you are not sure exactly what is triggering your symptoms — understanding your specific allergen sensitivities may be a useful next step. Identifying potential triggers can support more targeted symptom management and help you make informed decisions about avoidance strategies.
A specific IgE blood test measures whether your immune system has produced antibodies to particular allergens, such as grass pollen, tree pollen, certain foods, or animal dander. This information can be helpful in building a clearer picture of what you may be reacting to.
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.
We provide nurse-led venous blood collection for laboratory allergy blood testing in London. We do not prescribe medication or provide GP services. Test results may support discussions with your healthcare professional about managing your symptoms.
Safety and When to Seek Urgent Help
While antihistamines are generally well tolerated, it is important to be aware of situations that require immediate medical attention. The following symptoms may indicate a severe allergic reaction 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 about antihistamines — 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
How long do antihistamines take to work on an empty stomach?
Taking an antihistamine on an empty stomach may allow it to be absorbed slightly faster, potentially reducing the time to onset by a small margin. However, the difference is generally modest. Most antihistamines can be taken with or without food. Always follow the guidance on the patient information leaflet, as some specific products may have food-related instructions.
How long do antihistamines last?
Modern non-drowsy antihistamines such as cetirizine, loratadine, and fexofenadine typically last around 24 hours, which is why they are usually taken once daily. Older first-generation antihistamines such as chlorphenamine tend to last approximately four to six hours and may need to be taken more frequently. Always follow the dosing guidance on the product packaging.
Why do antihistamines help sneezing but not a blocked nose?
Antihistamines work by blocking histamine receptors, which makes them effective for symptoms directly driven by histamine — such as itching, sneezing, and a runny nose. Nasal congestion, however, is caused primarily by inflammation and swelling of the nasal tissues, which histamine blockers do not fully address. A corticosteroid nasal spray may be a more suitable option for congestion; a pharmacist can advise.
Can antihistamines stop itching straight away?
Most oral antihistamines begin to reduce itching within 30 to 60 minutes, though this varies between individuals. The level of relief also depends on the cause and severity of the itching. For faster localised relief, antihistamine creams or eye drops may act more quickly in the specific area where they are applied.
Can I take two antihistamines together?
Combining antihistamines should not be done without professional guidance. Taking two different types at the same time — or exceeding the recommended dose of one — can increase the risk of side effects, particularly drowsiness. If a single antihistamine is not providing adequate relief, speak to a pharmacist before making any changes to how you are taking your medicine.
Do antihistamines affect allergy blood test results?
Allergy blood tests measure specific IgE antibodies in the blood, and this measurement is not typically affected by antihistamine use. Unlike skin prick tests — which can be influenced by antihistamines — blood-based IgE testing can generally be carried out while you continue taking your usual antihistamine medicine. However, always follow any specific instructions provided at the time of booking your test.
References
- NHS — "Antihistamines" (NHS patient information page)
- NHS — "Hay fever" (NHS Health A to Z)
- NHS — "Hives (urticaria)" (NHS patient information)
- NICE Clinical Knowledge Summaries — "Allergic rhinitis: management"
- British National Formulary (BNF) — antihistamine monographs (general pharmacological reference)
- Allergy UK — "Treatments for allergy" (patient information)
- DVLA — guidance on medicines and driving fitness