What do antihistamines do for allergies

Common oral antihistamines include these categories:

  1. Over-the-counter (OTC): Probably the most well-known oral antihistamine is Benadryl, which is an older, sedating antihistamine.

    What do antihistamines do for allergies

    While Benadryl is a reasonable medication for the treatment of various allergic conditions, side effects limit its routine use, especially during the daytime. Benadryl, and numerous other older antihistamines, such as Chlor-Trimeton (chlorpheniramine), are available OTC without a prescription.

    What do antihistamines do for allergies

    Newer, less-sedating antihistamines, Claritin, Allegra, and Zyrtec, are also available OTC without a prescription. These medications cause much less sedation and are therefore preferred over the older, sedating forms.

  2. Prescription only: While numerous older, sedating antihistamines are now available OTC, Atarax is still only available by prescription. Atarax is extremely sedating and is commonly prescribed for the treatment of skin allergies such as hives and itching. The athletic metabolite of Atarax, Zyrtec, is available OTC and causes much less sedation. Xyzal and Clarinex, two less sedating antihistamines, are still available only by prescription, as is Periactin (cyproheptadine).

Other Uses for Antihistamines

Antihistamines are frequently used for the treatment of other allergic conditions, although they may not be as effective as they are for hay fever and hives.

These conditions may include:


Angioedema is dangerous if laryngeal oedema is present. In this circumstance adrenaline/epinephrine injection, oxygen, antihistamines and corticosteroids should be given as described under Anaphylaxis. Tracheal intubation may be necessary. In some children with laryngeal oedema, adrenaline 1 in 1000 (1 mg/mL) solution may be given by nebuliser. However, nebulised adrenaline/epinephrine cannot be relied upon for a systemic effect—intramuscular adrenaline/epinephrine should be used.

Hereditary angioedema

The treatment of hereditary angioedema should be under specialist supervision.

Unlike allergic angioedema, adrenaline/epinephrine, corticosteroids, and antihistamines should not be used for the treatment of acute attacks, including attacks involving laryngeal oedema, as they are ineffective and may delay appropriate treatment—intubation may be necessary. The istration of C1-esterase inhibitor (in unused frozen plasma or in partially purified form) can terminate acute attacks of hereditary angioedema; it can also be used for short-term prophylaxis before dental, medical, or surgical procedures. Tranexamic acid is used for short-term or long-term prophylaxis of hereditary angioedema; short-term prophylaxis is started several days before planned procedures which may trigger an acute attack of hereditary angioedema (e.g.

dental work) and continued for 2–5 days afterwards.

What do antihistamines do for allergies

Danazol [unlicensed indication] is best avoided in children because of its androgenic effects, but it can be used for short-term prophylaxis of hereditary angioedema.

Side Effects

Older antihistamines, such as Benadryl and Atarax, own a significant quantity of anticholinergic side effects, including dry mouth, drowsiness, constipation, headache, and urinary retention. Because of the side effects of these medications, they are generally considered to be too sedating for routine daytime use. Since older antihistamines can impair mental and motor functioning, they can decrease your ability to operate motor vehicles or heavy machinery.

Newer, low-sedating antihistamines, such as Claritin and Zyrtec, tend to own fewer anticholinergic side effects.

While these newer antihistamines may still cause drowsiness or a dry mouth, they haven’t been shown to impair your ability to operate a motor vehicle. Allegra is the only antihistamine that is truly considered to be non-sedating.


Generic Name:diphenhydramine (DYE fen HYE dra meen)

Medically reviewed by Sanjai Sinha, MD Final updated on Dec 18, 2018.

Weight Acquire

In addition to sedation and dry mouth, antihistamines may own the unwanted side effect of increased appetite and weight gain. This may be due to the similar chemical structure of antihistamines and certain psychiatric medications, such as anti-depressants, which are known to increase appetite and lead to weight gain.

In fact, numerous people who own used Xyzal reported an increase in appetite and weight acquire.

The package insert for Xyzal confirms weight acquire as a known side effect but reports this as only occurring in 0.5 percent of people in studies who were taking this medication. Older antihistamines, such as Periactin (cyproheptadine), own actually been used for the purpose of increasing appetite and weight acquire in underweight children and cancer patients undergoing chemotherapy.

In numerous states, you can be charged with driving-under-the-influence (DUI) if you operate a motor vehicle while taking medications such as Benadryl.


Anaphylaxis is a severe, life-threatening, generalised or systemic hypersensitivity reaction.

What do antihistamines do for allergies

It is characterised by the rapid onset of respiratory and/or circulatory problems and is generally associated with skin and mucosal changes; immediate treatment is required. Children with pre-existing asthma, especially poorly controlled asthma, are at specific risk of life-threatening reactions. Insect stings are a recognised risk (in specific wasp and bee stings). Latex and certain foods, including eggs, fish, cow’s milk protein, peanuts, sesame, shellfish, soy, and tree nuts may also precipitate anaphylaxis (see Food allergy). Medicinal products particularly associated with anaphylaxis include blood products, vaccines, allergen immunotherapy preparations, antibacterials, aspirin and other NSAIDs, and neuromuscular blocking drugs.

In the case of drugs, anaphylaxis is more likely after parenteral istration; resuscitation facilities must always be available for injections associated with special risk.

What do antihistamines do for allergies

Refined arachis (peanut) oil, which may be present in some medicinal products, is unlikely to cause an allergic reaction—nevertheless it is wise to check the full formula of preparations which may contain allergens.

Intramuscular adrenaline (epinephrine)

The intramuscular route is the first choice route for the istration of adrenaline/epinephrine in the management of anaphylaxis. Adrenaline/epinephrine is best given as an intramuscular injection into the anterolateral aspect of the middle third of the thigh; it has a rapid onset of action after intramuscular istration and in the shocked patient its absorption from the intramuscular site is faster and more dependable than from the subcutaneous site.

Children with severe allergy, and their carers, should ideally be instructed in the self-istration of adrenaline/epinephrine by intramuscular injection.

Prompt injection of adrenaline/epinephrine is of paramount importance.

The adrenaline/epinephrine doses recommended for the emergency treatment of anaphylaxis by appropriately trained healthcare professionals are based on the revised recommendations of the Working Group of the Resuscitation Council (UK).

Treatment of anaphylaxis

Adrenaline/epinephrine provides physiological reversal of the immediate symptoms associated with hypersensitivity reactions such as anaphylaxis and angioedema.

First-line treatment includes:

  1. istering an antihistamine, such as chlorphenamine maleate, by slow intravenous injection or intramuscular injection as adjunctive treatment given after adrenaline.
  2. securing the airway, restoration of blood pressure (laying the kid flat and raising the legs, or in the recovery position if unconscious or nauseous and at risk of vomiting);
  3. istering adrenaline/epinephrine by intramuscular injection; the dose should be repeated if necessary at 5-minute intervals according to blood pressure, pulse, and respiratory function;
  4. istering high-flow oxygen and intravenous fluids;
  5. istering an intravenous corticosteroid such as hydrocortisone (preferably as sodium succinate) is of secondary worth in the initial management of anaphylaxis because the onset of action is delayed for several hours, but should be given to prevent further deterioration in severely affected children.

Continuing respiratory deterioration requires further treatment with bronchodilators including inhaled or intravenous salbutamol, inhaled ipratropium bromide, intravenous aminophylline, or intravenous magnesium sulfate [unlicensed indication] (as for acute severe asthma); in addition to oxygen, assisted respiration and possibly emergency tracheotomy may be necessary.

When a kid is so ill that there is doubt about the adequacy of the circulation, the initial injection of adrenaline/epinephrine may need to be given as a dilute solution by the intravenous route, or by the intraosseous route if venous access is difficult; for details see adrenaline/epinephrine.

On discharge, kid should be considered for further treatment with an oral antihistamine and an oral corticosteroid for up to 3 days to reduce the risk of further reaction.

The kid, or carer, should be instructed to return to hospital if symptoms recur and to contact their general practitioner for follow-up.

Children who are suspected of having had an anaphylactic reaction should be referred to a specialist for specific allergy diagnosis. Avoidance of the allergen is the principal treatment; if appropriate, an adrenaline/epinephrine auto-injector should be given for self-istration or a replacement supplied.

Intravenous adrenaline (epinephrine)

Intravenous adrenaline/epinephrine should be given only by those experienced in its use, in a setting where patients can be carefully monitored.

Where the kid is severely ill and there is genuine doubt about adequacy of the circulation and absorption from the intramuscular injection site, adrenaline/epinephrine may be given by slowintravenous injection, repeated according to response; if multiple doses are required consider giving adrenaline by slow intravenous infusion.

It is also significant that, where intramuscular injection might still succeed, time should not be wasted seeking intravenous access.

Adrenaline/epinephrine is also given by the intravenous route for acute hypotension.

Most Common Uses of Antihistamines

Antihistamines are commonly used for the treatment of various allergic diseases, and can be taken in oral, nasal spray, eye drop, and injected forms.

The diseases antihistamines are used for include:

  1. Eye allergies: Oral antihistamines can assist treat the symptoms of eye allergies. They're also available as eye drops, such as Optivar (azelastine), Emadine (emedastine), Visine-A (pheniramine), and Alaway (ketotifen). Corticosteroid eye drops may be used to treat eye allergies when antihistamines don't work. Examples include Flarex (fluorometholone), Alrex (loteprednol), and Omnipred (prednisolone).
  2. Hay fever (allergic rhinitis): Newer oral antihistamines, such as Xyzal (levocetirizine), Zyrtec (cetirizine), Allegra (fexofenadine), Clarinex (desloratadine), and Clarinex (desloratadine), are extremely effective therapies for the treatment of hay fever. Older antihistamines, love diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), and hydroxyzine (Atarax), also work well but own numerous side effects.

    Oral antihistamines work well when they're taken as-needed, meaning that your allergy symptoms improve within an hour or two of taking the medication. Antihistamines are also available as nasal sprays for the treatment of allergic rhinitis (nasal allergies). Nasal antihistamine sprays include Astelin (azelastine) and Patanase (olopatadine). Corticosteroid nasal sprays are often used to treat allergies as well. Examples of these include Rhinocort (budesonide), Flonase (fluticasone propionate), Nasonex (mometasone), and Nasacort Allergy 24 Hour (triamcinolone).

  3. Hives (urticaria): Oral antihistamines are the main treatment for hives.

    What do antihistamines do for allergies

     Newer, low-sedating antihistamines, such as Zyrtec or Claritin, are preferred over the older, sedating antihistamines such as Benadryl or Atarax. Topical antihistamine creams, present in numerous forms of over-the-counter anti-itch creams, should not be used for the treatment of hives or other skin allergies because using topical antihistamines on the skin, such as Benadryl cream, can cause contact dermatitis due to the antihistamine.