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Oral Anti-allergy Medicines


Allergy is a condition when your body’s immune system overreacts to a substance that is usually harmless (allergen). The allergen may be inhaled, ingested, injected or contacted. There are many types of allergies, including atopic allergy (allergic rhinitis, eczema, and allergic conjunctivitis), food allergy (commonly caused by eggs, milk, seafood and nuts), insect sting allergy, drug allergy (most common allergens are penicillin-based antibiotics and vaccines) and environment allergy (pollen allergy and dust mite allergies).


Symptoms of allergies include runny nose, sneezing, watery and itchy eyes, skin itching, hives, swelling of the lips, face or tongue, etc. More serious reactions include swelling of the throat and airways, wheezing, and loss of consciousness.

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The most effective way of managing an allergy is by avoiding contact with the causative allergens. If this cannot be achieved, medications may be needed to relieve the symptoms. Commonly used anti-allergy medicines include antihistamines, corticosteroids, decongestants and montelukast. These medicines are available in many dosage forms, such as tablets, capsules, inhalers, topical creams, ointments and injections. In Hong Kong, most of the oral anti-allergy medicines can be bought in licensed medicine stores or pharmacies. However, you should seek advice from your family doctor or other healthcare professionals before taking any medicines.

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Oral Anti-allergy Medicines

(i) Antihistamines

Histamines are released when your body defends against attack. If you have an allergy, your immune system treats the allergen as an invader. Histamine release causes inflammation characterized by expansion of small blood vessels and swelling of surrounding skin or tissues. Antihistamines block the effects of histamine and thus relieve symptoms of allergy. Oral antihistamines are used for the treatment of a number of allergies, including allergic rhinitis in which they reduce runny nose and sneezing but are usually less effective for nasal congestion. They are also used to treat urticarial rash (hives), itchy skin and eyes (eczema and conjuctivitis). They are also of some value in preventing hives and in treating drug allergies.

  Anti-allergy Medicines

The term antihistamine is referred to medicines which act by blocking the histamine-1 receptor. They are classified as first generation (sedating) and second generation (non-sedating) antihistamines.

(a) Sedating Antihistamines

These are the older type of antihistamines and are characterized by their sedating and antimuscarinic effects. Classical examples of sedating antihistamines include brompheniramine, chlorpheniramine, cinnarizine, cyclizine, cyproheptadine, diphenhydramine, promethazine and triprolidine. Their sedative effect can vary from slight drowsiness to deep sleep. Diphenhydramine and promethazine are associated with more severe sedative effects and can be used for short-term sedation. Cyproheptadine is often associated with increase in appetite and weight gain.

(b) Non-sedating Antihistamines

Examples of non-sedating antihistamines include cetirizine, desloratadine, fexofenadine and loratadine. These are newer antihistamines and generally cause little or no drowsiness due to their poor penetration into the central nervous system.

(ii) Corticosteroids

The role of oral corticosteroids in the treatment of allergies is reserved only for severe cases, such as severe attacks of hives non-responsive to standard treatment and resistant severe eczema. (Please refer to the following link of the previous article ‘Information on oral corticosteroid for more detail: )

(iii) Decongestants

Decongestants relieve nasal congestion and thus provide symptomatic relief to allergic rhinitis and hay fever. They reduce swelling of blood vessels inside your nasal cavity in the upper airway to make breathing easier. However, use of decongestants for more than 7 days can lead to rebound congestion and therefore they are not recommended to be used regularly or for long term. Examples of commonly used decongestants include pseudoephedrine and phenylephrine.

(iv) Montelukast

Montelukast, a leukotriene antagonist, has been used for the management of allergic rhinitis. Montelukast blocks the effect of potent inflammatory substances released from cells including mast cells and eosinophils, thus improving symptoms of allergic rhinitis, such as nasal congestion, rhinorrhea, sneezing and nasal itching.


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Common Side effects and Precautions of Oral Anti-allergy medicines

Types of Anti-allergy Medicines

Common side effects


(i) (a) Sedating antihistamines

  • Drowsiness, dizziness, lassitude
  • Dry mouth, thickened respiratory tract secretions, blurred vision, constipation, urinary retention
  • Nausea, vomiting and epigastric pain
  • Headache

  • 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

(b) Non-sedating antihistamines

  • Palpitations and arrhythmias

  • 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

(ii) Corticosteroids

  • 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

  • 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

(iii) Decongestants

  • Rise in blood pressure, heart rate
  • Increased alertness which may lead to difficulty in falling asleep if taken during the latter part of the day.

  • Caution with use in patients with high blood pressure and glaucoma

(iv) Montelukast

  • Headache, abdominal pain
  • Hyperactivity in children
  • Hypersensitivity
  • Dry mouth, stomach upset
  • Elevated levels of liver enzymes

  • Very rarely, Churg-Strauss syndrome may occur, presenting with clinical features of blood vessel inflammation


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General advice on taking anti-allergy medicines

  • Some anti-allergy medicines can cause drowsiness or impair mental alertness. Do not drive or operate machinery if you feel drowsy or your mental alertness is impaired after taking your anti-allergy medicines.

  • Be familiar with the name and dosage of the medicines you are taking. Be cautious of their possible side effects

  General advice

  • Do not drink alcohol while taking your medicines as this will increase the sedating effects of antihistamines or increase the risk of other side effects.

  • For patients who have high blood pressure, glaucoma and asthma, seek advice from your family doctor or pharmacist before using any anti-allergy medicines.

  • Anti-allergy medicines do not provide cure but merely relieve symptoms. If the allergen is known, the best way to manage your allergy is to avoid contact with the causative source


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Communication with your doctor or pharmacist

  • Seek your family doctor or pharmacist’s advice for the best treatment option. You may be advised to use non-medicinal products or other measures to relieve your allergies, such as:

    • using non-medicated bath oils for soothing skin symptoms;

    • using saline nasal rinse for the relief of nasal symptoms;

    • wearing gloves or applying barrier creams to avoid direct skin contact with allergens in cases of contact dermatitis.

  • Inform your family doctor or pharmacist of the medicines you are taking as your medicines may interact with the anti-allergy drugs. You should also inform them of any diseases you are suffering as some diseases may warrant special precautions.

  • Seek advice from your family doctor if you suspect you are experiencing side effects from your anti-allergy medicines that are persistent and affect your daily life.

  • Always ask your family doctor before taking any other medicines or health products as they may affect the effect or increase the side effects of your medication.

  Communication with your doctor


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Storage of medicines

Anti-allergy medicines should be stored in a cool dry place. Unless specified on the label, anti-allergy medicines should not be stored in refrigerators. Furthermore, anti-allergy medicines should be kept properly in places unreachable by children to prevent accidental ingestion.



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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.


Drug Office
Department of Health
Nov 2012