Antihistamines: Uses, Types, and Side Effects of a Popular Allergy Medication

Antihistamines are medications that help relieve or prevent allergy symptoms. Most are available over-the-counter (OTC) and come in the form of tablets, liquid, nasal sprays or gels, and eye drops. As the name suggests, antihistamines reduce or block histamine, a chemical produced by the body during an allergic reaction.

Antihistamine Uses and Indications

Antihistamines are mainly indicated to treat allergic reactions, but can also be used as sedatives, and in the treatment and prevention of motion sickness and drug-induced Parkinsonism. Their effect is short-lived, providing symptomatic relief only; they do not correct the underlying cause.

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What Are Allergies?

Allergies occur when your body recognizes a substance or “allergen” as foreign. The allergic reaction is a complex process involving the release of histamine, a chemical that causes symptoms such as repetitive sneezing and itchy, watery eyes—our body’s reaction to get rid of the allergen.

Histamine can also cause a skin rash called “hives” (urticaria or nettle-rash). Sometimes the body overreacts to substances in the environment, in food, or on skin, causing unpleasant, ongoing symptoms. Antihistamine medications can reduce some of the allergy symptoms, but may not clear them all.

Types of Antihistamines

Experts at the American Academy of Allergy, Asthma, & Immunology (AAAAI) explain that antihistamines are divided into:

• “First generation/sedating,” which may cause drowsiness
• “Second generation/low-sedating or non-sedating,” which are much less likely to cause drowsiness.

First-Generation Antihistamines

First generation antihistamines not only block histamine but also work in the part of the brain that controls nausea and vomiting, so they can help prevent motion sickness. However, they often cause drowsiness and so should not be taken when driving or operating machinery. Their sedative effect leads to them being used as a sleep aid alone, or in combination with other drugs in nighttime cold-and-flu formulations. Examples include brompheniramine (Dimetapp), diphenhydramine (Benadryl, Nytol, Sominex), and doxylamine (NyQuil).

Potential side effects of first-generation antihistamines include dry mouth, dizziness, nausea, muscle weakness, drowsiness, constipation, and trouble passing urine (in men with prostate disease).

In children potential side effects may include nightmares, restlessness, and irritability. Sometimes parents use these medications to sedate children before a flight or stressful event, such as vaccinations, but be warned: Children could react in the opposite way and become increasingly alert and agitated. Test your child’s reaction on at least one occasion before the main event and never give more than the recommended dose for the child’s age and size. There have been deaths from these medications in children.

Second-Generation Antihistamines

These newer second-generation antihistamines have a more specific action (on H1 receptors) and so provide relief from allergy symptoms with less sedation and other side effects. Common types include loratadine (Claritin), ctirizine (Zyrtec), and fexofenadine (Allegra). Side effects are rare, but about 10 percent of people report headaches.

They work well as a first-line therapy for allergic rhinitis (sneezing, runny nose, and itching). They do not reduce congestion, so are often combined with other drugs in, for example, cold and flu remedies. Read the labels to ensure you don’t inadvertently double up. They can be taken as needed, or daily for persistent symptoms.

Precautions with Antihistamines

As with any drug, read and follow the directions for antihistamines on the label or insert.

It is recommended that you ask your doctor or pharmacist whether it’s okay for you to take antihistamines, especially if you are on multiple medications or if you have:

• Glaucoma
• Enlarged prostate or problems passing urine
• Epilepsy
• Overactive thyroid
• Heart disease or high blood pressure
• Diabetes
• Liver or kidney disease
• Respiratory problems such as asthma and chronic obstructive pulmonary disease (COPD)

Also, check with your doctor if you are pregnant or nursing.

Antihistamines for Children

The U.S. Food and Drug Administration (FDA) advises that children as young as 6 months can take some antihistamines, but they warn: “Always read the label to make sure the product is right for your child’s age… [Also] read the label to be sure that the active ingredients aren’t the same. Children are more sensitive than adults to many drugs.”

With babies and children, it is best to err on the side of caution and see your doctor if your child develops new symptoms, as allergies can mimic more serious illness. If your child has been diagnosed with allergies, follow the doctor’s medication instructions with care.

Alternatives to Antihistamines for Allergy Treatment

• Avoid the allergen. If at all possible, reduce your exposure to allergens. For example: Avoid dogs if you’re allergic to dog hair; dust and vacuum bedrooms meticulously if you have a dust mite allergy; and wash your hair before bed if you have hayfever and have been outside during the day.
• Take nasal corticosteroids. These come in the form of a nasal spray and are taken once or twice a day to reduce inflammation and nasal symptoms. Side effects may include stinging in the nose.
• Take Decongestants. These come in oral and nasal spray forms and are available by prescription or OTC. They are sometimes used with antihistamines. Examples include oxymetazoline (Vick’s Sinex) and pseudoephedrine (Sudafed). Pseudoephedrine is available without a prescription but is kept behind the pharmacy counter, as it can be made into methamphetamine, an illegal and highly addictive stimulant. One problem with decongestants is that they work for a short time and then can result in a “rebound” effect,” where the congestion gets worse.
• Try immunotherapy. For people with severe allergies, allergy shots or tablets may be indicated, ask your allergy doctor for information.
• Use natural products. The National Center for Complementary and Integrative Health (NCCIH) provides evidence that the following may be beneficial in treatment of allergies: Saline nasal irrigation, butterbur extract, acupuncture, and probiotics.

A Note on Severe Allergies

Antihistamines are unlikely to be effective in the case of a severe allergic reaction, known as anaphylaxis.

Symptoms that suggest a severe reaction requiring urgent medical attention include sudden onset of:

• Shortness of breath, coughing, and wheezing
• Tightness in chest
• Hoarseness or constriction in throat
• Swelling in tongue, lips, face, or throat
• Severe itching or widespread hives
• Swaeting or flushing
• Abdominal pain with nausea, vomiting or diarrhea
• Chest pain
• Dizziness or fainting

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