Antihistamines are frequently prescribed against allergies, especially against allergy to pollen. How do they work? What are their limits… and their drawbacks? Are over-the-counter antihistamines as effective? We take stock with two specialists.
What exactly is an antihistamine? It’s a medicine that blocks the effects of histamine, a substance released by cells when you have allergic reaction. It is effective against certain symptoms of an allergic crisis: itching, runny nose, weeping eyes, sneezing… but absolutely not on allergic asthma. People who are allergic to pollen often use antihistamines to relieve their symptoms.
In what form to take them? If the allergic reaction is isolated, at the level of the nose or the eyes, the local forms (an eye drops or a nasal spray) have a much faster action than the tablets (around 2 minutes for the eye drops). However, their effect is limited in time. This requires renewing the application two or three times a day. Conversely, most second-generation systemic antihistamines (tablets), which also act on local symptoms, are active for 12 to 24 hours.
Are non-prescription antihistamines less effective?
Absolutely not : these are the same products! But they are sold in simplified packaging (for example cetirizine, Humex® allergy, Reactine®, Zyrtecset®, etc.). However, you should not buy an antihistamine you are not used to. Even if they have few side effects, some are contraindicated in pregnancy (info on www.lecrat.org). Others are in cases of kidney failure, glaucoma (increased pressure in the eye) or prostate adenoma. They are not harmless drugs. The instructions should therefore be read carefully, especially if you are taking other drugs, certain combinations being to be avoided.
What are the contraindications for antihistamines?
The latest antihistamines on the market (Bilaska®, Inorial®) and available on prescription cause less drowsiness than others. However, a few people still experience these effects! Depending on the case, these antihistamines also exhibit interactions with certain drugs (erythromycin, ketoconazole, etc.). And, as a precaution, they are not recommended during pregnancy. Some should also be taken away from meals and without grapefruit juice, which decreases their effectiveness by 30%.
What are their side effects?
Unlike the first generation antihistamines (the anticholinergic antihistamines) which used to cause drowsiness, the newer antihistamine drugs have little or no sedating effect on most people. But, whatever the form used, it is nevertheless necessary to remain cautious, in particular in the event of a risky situation (if you have to drive a vehicle for example). People who still feel drowsy should take their tablet in the evening before bedtime and not in the morning.
When should I take antihistamines?
If the allergic rhinitis is moderate, antihistamines may be sufficient to relieve symptoms. They are then taken every day during the period of exposure to pollen. But if the rhinitis is associated with asthma or another respiratory allergy (dust mites, etc.), or if the symptoms do not cease with antihistamines alone, other treatments may be offered. As a combination with corticosteroids, or even desensitization. But this requires taking stock of your allergic status with an allergist.
Should we start before the arrival of pollens?
This is unnecessary, since it is the presence of pollens that triggers the allergic crisis. And antihistamines only work on symptoms. However, there are preventive local treatments based on cromones, substances which have a protective barrier effect and reduce hypersensitivity reactions in cases of rhinitis or allergic conjunctivitis.
Are they effective against food allergies?
It all depends on the type of symptoms caused by contact with the allergen. If these are commonplace and the person has never had a serious reaction, antihistamines may be sufficient. But if the signs persist, they must alert and seek consultation as soon as possible. In addition, if there is a history of anaphylactic shock (swelling, malaise, etc.), their effect on the symptoms may mask the onset of the attack. However, this is a sign indicating that an injection of adrenaline must be given urgently.
Do they prevent you from having other allergies?
In any way, since antihistamines are only symptomatic treatments. They do not act on the evolution of the allergy: eczema, food or respiratory allergy, asthma… In fact, only desensitization seems to reduce the risk of developing new allergies and prevent the onset of asthma. This has also been demonstrated in children, in particular with desensitization to birch and grass pollens.
What to do when antihistamines stop working?
In general, if antihistamines are no longer as effective, the allergic rhinitis has progressed and chronic inflammation has developed. In chronic rhinitis, the nose becomes autonomously hyperactive, and antihistamines have less effect. After an allergological assessment, the doctor can then prescribe other drugs, such as corticosteroids in nasal spray, or eye drops containing cromones in case of conjunctivitis. It can also offer desensitization, if possible, in order to gradually re-accustom the body to the responsible allergen.
But desensitization is only recommended in three situations:
- if the allergic reactions persist despite proper treatment
- if the patient becomes asthmatic
- if it does not support local treatments.
Antihistamines can be taken during desensitization, but this is rarely necessary. Allergic reactions have become exceptional with sublingual desensitization, which is increasingly used (80% of cases).
If the allergic reaction persists or gets worse, can it be resumed?
No. If you have an allergic reaction and antihistamines have not worked or symptoms worsen, emergency procedures should be followed. Call for help (by dialing 15), then take a dose of bronchodilator in the event of difficult breathing or an asthma attack (to be repeated every 5 minutes while waiting for help). An injection of adrenaline is necessary in the event of the appearance of general signs: discomfort, difficulty in breathing, intense abdominal pain, edema, hoarse cough … Followed by another in the absence of response 5 to 10 minutes after the first injection .
Our experts
Dr Dominique Château-Waquet, allergist in Paris
Prof. Frédéric de Blay, pneumo-allergist, Strasbourg university hospitals
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