Antihistamines represent one of the main allies in the fight against allergic symptoms, but their use requires special attention, especially when it comes to children of different ages or pregnant women.
In this comprehensive guide, we will explore everything there is to know about these drugs: from their mechanism of action to the different types available, to the specific precautions for the most sensitive categories.
What are antihistamines, and how do they work?
Antihistamines are drugs that block the action of histamine, a substance that our body releases during allergic reactions.
When an allergen (such as pollen, dust, or animal hair) comes into contact with our body, the immune system mistakenly identifies it as a threat and releases histamine from mast cells, cells present in connective tissues.
Histamine binds to specific receptors on cells, causing the classic allergy symptoms: dilation of blood vessels (which causes redness and swelling), increased capillary permeability (which leads to edema), contraction of smooth muscle (which can cause bronchospasm) and stimulation of the mucous glands (resulting in rhinorrhea and tearing).
Antihistamines work by occupying histamine receptors, preventing histamine from binding and thus blocking the cascade of events that leads to allergy symptoms. It is important to emphasize that antihistamines do not stop the release of histamine, but prevent its effects.
The different generations of antihistamines
Over the years, pharmaceutical research has developed different generations of antihistamines, each with specific characteristics:
First-generation antihistamines
The first antihistamines developed include:
- Diphenhydramine (Benadryl)
- Clemastine (Tavegil)
- Hydroxyzine (Atarax)
- Promethazine (Farganesse)
These drugs have some significant disadvantages:
- They easily cross the blood-brain barrier, causing drowsiness in 50-60% of patients
- They can interfere with cognitive and psychomotor performance
- They have a short duration of action (4-6 hours), requiring multiple administrations during the day
- They can cause anticholinergic effects such as dry mouth, urinary retention, and blurred vision
Despite these limitations, in some specific cases they can be useful, for example when the sedative effect is desirable (allergies that disturb sleep).
Second-generation antihistamines
Developed since the 1980s, they include:
- Cetirizine (Zirtec)
- Loratadine (Clarityn)
- Desloratadine (Aerius)
- Fexofenadine (Telfast)
- Bilastine (Labixten)
The advantages of these drugs are significant:
- Minimal penetration of the blood-brain barrier, resulting in a significant reduction in drowsiness
- Prolonged duration of action (12-24 hours), allowing a single daily administration
- Absence or reduction of anticholinergic effects
- Absence of significant interactions with food or other drugs (with some exceptions)
For these reasons, second-generation antihistamines are now considered the drugs of first choice for the treatment of allergies in almost all age groups.
Antihistamines and children: dosages and precautions by age group
The use of antihistamines in children requires special attention, with dosages that vary significantly based on age and weight. Let’s look at the main considerations for each age group:
Newborns and infants (0-12 months)
In this age group, the use of antihistamines is generally not recommended, except in exceptional cases and under strict medical supervision. Reasons include:
- Immature hepatic enzyme system, which can alter the metabolism of the drug
- Increased sensitivity to side effects
- Difficulty in recognizing and reporting any adverse reactions
If necessary, only a specialist pediatric allergist should prescribe antihistamines in this age group, favoring specific formulations and very cautious dosages.
Young children (1-5 years)
For children between 1 and 5 years:
- Second-generation antihistamines are preferable for their better safety profile
- Liquid formulations (syrups or drops) facilitate administration and allow for more precise dosing
- Cetirizine is often the drug of first choice, with dosages that can vary from 2.5 mg to 5 mg per day, depending on age and weight
- It is important to carefully monitor any side effects, particularly irritability or behavioral changes
Some children in this age group may manifest the paradoxical effect of antihistamines, with agitation rather than sedation, especially with first-generation drugs.
School-age children (6-12 years)
For children between 6 and 12 years:
- Dosages progressively approach those of adults, but generally remain lower
- Formulations may include orodispersible tablets, which are easier to take
- Loratadine and cetirizine are generally well tolerated, with dosages of 5-10 mg and 5-10 mg per day respectively
- It is important to consider the impact of first-generation antihistamines on school performance, avoiding them during the school period
In this age group, it is possible to begin educating the child about self-management of allergy, teaching them to recognize symptoms and the importance of adherence to therapy.
Adolescents (13-18 years)
For adolescents:
- Dosages are generally equivalent to those of adults
- It is important to consider possible interactions with other drugs that the adolescent might be taking
- Communication about the correct use of the drug becomes fundamental, explaining the importance of not exceeding prescribed dosages
- Second-generation antihistamines are preferable to minimize the impact on school and sports activities
Adolescents can be particularly sensitive to the aesthetic impact of allergy symptoms (red eyes, runny nose), which can increase motivation for therapeutic adherence.
Side effects and interactions to monitor in children
Even second-generation antihistamines, although generally safe, can cause side effects in children:
- Dry mouth (more common)
- Fatigue or mild drowsiness (in a smaller percentage compared to adults)
- Headache
- Gastrointestinal disorders such as nausea or abdominal pain
- Rarely, agitation or irritability
It is important to inform the pediatrician of any undesirable effects observed after starting antihistamine therapy.
Regarding drug interactions, the main second-generation antihistamines have a low potential for interaction, but it is always good to inform the doctor if the child is taking other drugs, in particular:
- Macrolide antibiotics
- Azole antifungals
- Antiepileptic drugs
- Corticosteroids
Antihistamines in pregnancy: risks, benefits and recommendations
Managing allergies during pregnancy represents a particular challenge, as it is necessary to balance the control of maternal symptoms with safety for the developing fetus.
Classification of antihistamines in pregnancy
The American FDA (Food and Drug Administration) has historically classified drugs into pregnancy risk categories, from categories A (controlled studies have not shown risks) to X (demonstrated risk). According to this classification:
- Most second-generation antihistamines fall into category B (no evidence of risk in animal studies, but lack of controlled studies in humans) or C (adverse effects in animal studies, but potential benefits that might justify the risk).
- Loratadine and cetirizine are generally considered among the safest options, with extensive clinical experience supporting their use.
Obviously, as always, this decision should be made with your doctor in conjunction with your gynecologist.
Recommendations by trimester of pregnancy
First trimester (weeks 1-12):
- During this period organogenesis occurs, when the organs of the fetus are forming
- It is preferable to avoid any medication if possible, favoring non-pharmacological measures
- If treatment is necessary, consult an allergist who, in collaboration with the gynecologist, can recommend the antihistamine with the best risk-benefit profile
Second trimester (weeks 13-26):
- With the completion of organogenesis, teratogenic risks decrease
- Second-generation antihistamines, particularly loratadine and cetirizine, can be considered if necessary
- Nasal corticosteroids represent an alternative with minimal systemic exposure
Third trimester (weeks 27-40):
- In the last months of pregnancy, it is important to consider potential effects on labor and the newborn
- Some first-generation antihistamines can theoretically affect uterine contractions
- It is preferable to maintain the therapy that has proven effective and well tolerated in previous trimesters, avoiding last-minute changes
Breastfeeding and antihistamines
During breastfeeding, small amounts of antihistamines can pass into breast milk. Recommendations include:
- Loratadine and cetirizine are considered the drugs of first choice during breastfeeding
- First-generation antihistamines can cause irritability or drowsiness in the infant, so they are generally not recommended
- It is preferable to take the drug immediately after a feeding and then wait 3-4 hours before the next feeding, to minimize exposure
Strategies to optimize the effectiveness of antihistamines
To maximize the benefits of antihistamine therapy:
- Optimal timing: Take antihistamines regularly, not only when symptoms are already present. The preventive effect is more effective than the curative one.
- Consistency: During allergy seasons, daily administration is more effective than as-needed.
- Anticipation: Starting therapy 1-2 weeks before the expected start of the pollen season can significantly reduce the intensity of symptoms.
- Combination with other therapies: In case of predominant nasal symptoms, combination with nasal corticosteroids can significantly improve effectiveness.
- Monitoring: Keeping a symptom diary can help evaluate the effectiveness of treatment and plan therapy for subsequent years.
When antihistamines are not enough: therapeutic alternatives
In some cases, antihistamines alone may not be sufficient to control allergy symptoms. Alternatives or complementary therapies include:
- Nasal corticosteroids: particularly effective for nasal congestion, which responds less to antihistamines
- Antileukotrienes: drugs such as montelukast, especially useful in case of associated allergic asthma
- Cromons: drugs such as sodium cromoglicate, available as nasal sprays or eye drops
- Specific immunotherapy: The “anti-allergy vaccine”, the only treatment capable of modifying the natural course of allergic disease
In more severe cases, especially in children with allergic asthma, it may be necessary to consider biological therapies such as anti-IgE antibodies, but only under strict specialist supervision.
It is important to know what you are doing.
Antihistamines represent a valuable resource in the management of allergies, but their effectiveness depends on careful and personalized prescription, taking into account the patient’s age, severity of symptoms, comorbidities and, in pregnant women, the trimester of gestation.
The choice of the most suitable antihistamine, the optimal formulation and the correct dosage regimen should always be guided by a doctor, ideally an allergist, who will know how to evaluate the risk-benefit ratio in each specific situation.
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We at Parentalife are by your side in the process of raising your children. We support parents with practical guides, and with personalized consultations.
We believe that informed parents raise more serene and healthy children, and our commitment is to provide you with accurate, up-to-date and scientifically valid information to accompany you on this wonderful journey.
SOURCES
Ten Eick AP, Blumer JL, Reed MD. Safety of antihistamines in children. Drug Saf. 2001;24(2):119-47. doi: 10.2165/00002018-200124020-00003. PMID: 11235817.
Pali-Schöll I, Motala C, Jensen-Jarolim E. Asthma and allergic diseases in pregnancy a review. World Allergy Organ J. 2009 Mar;2(3):26-36. doi: 10.1186/1939-4551-2-3-26. PMID: 211518