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Oral Anti-allergy Medicines
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| Allergies |
Allergy is a condition in which the body’s immune system overreacts to a foreign substance that is usually harmless in most people. These substances are known as allergens. There are various types of allergies, including:
- Atopic allergy: such as allergic rhinitis, eczema, allergic conjunctivitis
- Food allergy: commonly triggered by eggs, milk, seafood and nuts
- Insect sting allergies
- Drug allergies: penicillin-based antibiotics are among the most frequently reported triggers
- Environmental allergies: often caused by pollen and dust mites.
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Allergic symptoms can range from mild to life-threatening. Mild to moderate symptoms include runny nose, sneezing, watery and itchy eyes, skin itching, hives, swelling of the lips, face or tongue, etc., and severe allergic reactions may include swelling of the throat and airways, wheezing, and loss of consciousness. |
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| Treatment |
The most effective way to manage allergies is to avoid contact with the triggering allergens whenever possible. If avoidance is not feasible, medications may be required to relieve the symptoms. Commonly anti-allergy medicines include antihistamines, corticosteroids, decongestants and montelukast. These medicines are available in different dosage forms, such as tablets, capsules, inhalers, nasal sprays, eye drops, topical creams, ointments and injections. It is important to consult your doctor or pharmacist before taking any medication to ensure its suitability and safety. |
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Oral Anti-allergy Medicines
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(i) Antihistamines
Histamines are released by the immune system in response to perceived threat. In allergic reactions, the immune system mistakenly identifies harmless substances as harmful, triggering the release of histamines. Antihistamines work by blocking histamine-1 (H1) receptors, thereby relieving allergy symptoms. They are effective in treating allergic rhinitis (e.g. reducing runny nose and sneezing), though less effective for nasal congestion). They are also used to treat urticarial rash (hives), itchy skin and eyes (eczema and conjunctivitis). They are also of some value in preventing hives and in treating drug allergies.
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Antihistamines are generally classified into two main groups: sedating (or first-generation) and non-sedating (or second- and third-generation) antihistamines.
(a) Sedating Antihistamines
Sedating antihistamines are also widely known as first-generation antihistamines. They are the older class of antihistamines. Classical examples of sedating antihistamines include brompheniramine, chlorpheniramine, cinnarizine, cyproheptadine, diphenhydramine, and promethazine.
(b) Non-sedating Antihistamines
Non-sedating antihistamines produce less sedation and are generally less likely to cause drowsiness than sedating antihistamines, as they penetrate the blood brain barrier only to a slight extent. Examples of non-sedating antihistamines include cetirizine, desloratadine, fexofenadine, levocetirizine and loratadine.
(ii) Corticosteroids
The role of oral corticosteroids in the treatment of allergic reactions is only reserved for short-term use during severe cases, such as severe attacks of hives non-responsive to standard treatment and resistant severe eczema. You may wish to refer to ‘Information on oral corticosteroids’ of the Drug Office website for more information.
(iii) Decongestants
Decongestants are a type of medicine that helps to provide temporary relief for nasal congestion. They work by reducing the swelling of the blood vessels inside your nasal cavity, which helps to open the airways and allow a person to breathe more easily. However, use of decongestants should be limited to short-term use for a few days only, usually for not more than 7 days. Examples of commonly used decongestants include pseudoephedrine and phenylephrine.
(iv) Montelukast
Montelukast, a leukotriene receptor antagonist, is used in the management of allergic rhinitis. Montelukast works by blocking inflammatory substances called leukotrienes, thereby reducing inflammation and help alleviate symptoms of allergic rhinitis, such as nasal congestion, rhinorrhea, sneezing and nasal itching.
Montelukast is usually prescribed and reserved for patients with allergic rhinitis who do not respond well to or cannot tolerate other treatments. Although neuropsychiatric side effects have been reported with montelukast use, they are rare. For mild allergy symptoms, other medicines are often sufficient and may be preferable as a first choice.
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Common Side Effects and Precautions of Oral Anti-allergy Medicines
Types of Anti-allergy Medicines |
Common side effects*
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Precautions* |
(i) (a) Sedating antihistamines
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- Drowsiness, dizziness, lassitude
- Dry mouth, thickened respiratory tract secretions, blurred vision, constipation, urinary retention
- Nausea, vomiting and epigastric pain
- Headache
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- Patients should not drive or operate machinery after drug intake
- Avoid alcohol
- Caution with use in asthmatic patients
- Seek medical advice if you feel your heart beats faster or irregularly after taking your medicines
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(b) Non-sedating antihistamines
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- Palpitations and arrhythmias
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- Patients should be wary of the possible sedative effect and should not drive or operate machinery if they feel drowsy
- Avoid alcohol
- Seek medical advice if you feel your heart beats faster or irregularly after taking the medicines
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(ii) Corticosteroids
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- Sodium and water retention, high blood pressure
- Cushing’s syndrome (typical features include Moon face, abdominal distension)
- Stomach upset
- Increased susceptibility to infections
- Osteoporosis when used long term
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- Contraindicated in patients with acute infections uncontrolled by antibiotic therapy
- Caution with use in patients with history of cardiovascular diseases, diabetes mellitus, hepatic or renal impairment
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(iii) Decongestants
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- Rise in blood pressure and heart rate
- Increased alertness which may lead to difficulty in falling asleep if taken during the latter part of the day.
- Anxiety
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- Caution with use in patients with high blood pressure and glaucoma
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(iv) Montelukast
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- Headache
- Abdominal pain
- Upper respiratory infection
- Skin reactions
- Dry mouth
- Stomach upset
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- Serious neuropsychiatric (NP) events have been reported with the use of montelukast. Because of the risk of NP events, the benefits of montelukast may not outweigh the risks in some patients, particularly when the symptoms of disease may be mild and adequately treated with alternative therapies.
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| General Advice on Taking Anti-allergy Medicines* |
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Some anti-allergy medicines can cause drowsiness or impair mental alertness. Therefore, do not drive or operate machinery if you feel drowsy or mentally impaired after taking these medications. Always be aware of how your medicine will affect you before engaging in activities that require full alertness.
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Be familiar with the name and dosage of the medicines that you are taking, and be cautious of their potential side effects
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If you have high blood pressure, glaucoma or asthma, consult your doctor or pharmacist before using any anti-allergy medicines, as some anti-allergy medications may worsen these conditions or interact with your current medications.
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Anti-allergy medicines do not provide a cure, they merely provide temporary symptomatic relief. If the allergen is known, the most effective way to manage your allergy is to avoid contact with the causative source.
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| Communication with Your Doctor or Pharmacist |
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Consult your doctor or pharmacist to determine the best treatment option. They may recommend non-medicinal products or measures to relieve your allergies, such as:
- using non-medicated bath oils to soothe skin symptoms;
- using saline nasal rinse to relieve nasal symptoms;
- wearing gloves or applying barrier creams to avoid direct skin contact with allergens in cases of contact dermatitis.
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Storage of medicines
Oral anti-allergy medicines should be stored in a cool dry place. Refrigeration is generally not required unless specified on the drug labels. Always keep medications out of reach and sight of children to prevent accidental ingestion.
* This information may not cover all possible side effects, precautions, or medical advice. Always consult your healthcare professional for guidance.
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Acknowledgement : The Drug Office would like to thank the Surveillance and Epidemiology Branch (SEB) and the Professional Development and Quality Assurance (PD&QA)
for their valuable contribution to the preparation of this article.
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Drug Office
Department of Health
December 2025
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