Antihistamines: What are they used for? How are they taken? Effectiveness How do they work? Types and Side Effects

It is a group of drugs that act by blocking the action of the chemical called histamine in the body.

These drugs can block histamine H1 or H2 receptors, but the group commonly known as antihistamines blocks the H1 receptor. They have several uses, but they are used more frequently to treat allergies.

What are Antihistamines used for?

If you are one of the many people who get hay fever every year, you will know all about antihistamines. This is one of the most common uses of antihistamines. They can also be used for several other problems. For example:

  • Urticaria (urticaria).
  • Itching (pruritus)
  • Feeling sick (nausea) and being sick (vomiting).
  • Severe allergic reactions (anaphylaxis).

Where can I get them?

Some of these medications are available to buy without prescription from your pharmacist, others are only available by prescription.

How do I take them?

These medications come in a variety of forms, as mentioned above.

Your doctor or pharmacist will advise you on how to take your medication, including what dose and how often. Read the brochure that comes with your particular brand to get more information.

How fast do antihistamines work?

An antihistamine tablet usually begins to work within 30 minutes after the shot. The efficacy peak is usually 1 to 2 hours after taking.

How long is the treatment needed?

This may vary according to the reason for treating it. If you have hay fever, you can take the medication during the pollen season.

How effective are they?

In general, it is possible to find an antihistamine that keeps your symptoms controlled.

Sometimes you may need to experiment and try more than one to find the one that works best for you.

In the case of hay fever, sometimes, if the antihistamine tablet is not enough on its own, you may also need eye drops and / or a nasal spray to completely fix all of your symptoms.

Antihistamines are generally more effective when taken consistently than intermittently. This is particularly applicable for people with hay fever (seasonal allergic rhinitis).

In the summer months, the pollen count is generally higher and may be in contact with the allergen on a regular basis. Taking the medication regularly can help keep your symptoms under control.

Its effectiveness will also depend on the dose you take and how the medication is administered.

Can I take them if I am pregnant or breast-feeding?

Pharmaceutical companies are, of course, quite reluctant to try medications in pregnant or lactating women, so there are no studies to guide them.

They tend to discourage their use to be safe, although there is no evidence that they cause a problem.

Discuss this with your doctor, who will explain the options. If the benefits of the treatment are thought to outweigh any possible risk, Loratadine is generally recommended.

What conditions are treated with these medications?

Antihistamines are commonly used:

  • To relieve symptoms associated with hay fever (seasonal allergic rhinitis). These may include:
  • Inflammation of the nose and eyes (rhinitis and conjunctivitis).
  • Sneezing
  • Itching in the eyes, nose and throat.
  • Nasal congestion (Rhinorrhea).
  • To reduce the severity of rash and itching associated with nettle-type rashes, such as hives (hives) and generalized itching (pruritus).
  • To help with rash and itching after bites or insect bites.
  • To prevent motion sickness and other causes of discomfort (nausea).
  • Occasionally to treat severe morning sickness during pregnancy.
  • In the care of the terminally ill, for its sedative and antiescaras effects.
  • In the emergency treatment of severe allergic reactions.

How do they work?

Histamine is a chemical produced naturally by several cells in your body.

It has a variety of different functions. Large amounts of histamine are produced in cells called mast cells, in places where the body comes into contact with the outside environment. For example, in the nose, throat, lungs and skin.

Here, mast cells and histamine are part of your immune defense system. (While, in the stomach, the histamine produced by the cells lining the stomach helps produce acid for the digestion of food).

The cells of your immune system control your blood and mucous to detect anything (for example, germs such as bacteria or viruses) that has not been produced by your body. (The mucous membranes are membranes that line the cavities of the body, such as the mouth, nose and digestive tract).

If your skin is damaged or if your immune system detects a foreign substance, histamine is released from the mast cells. Histamine binds to special sites (receptors) in other cells, called H1 receptors.

This triggers a chain reaction that causes the blood vessels in the area to become a little permeable. Specialized cells and chemicals, which defend your body, can now access the area.

While this is a useful answer, it also causes redness, swelling and itching. Allergic reactions such as hay fever (seasonal allergic rhinitis)

These are caused by a hypersensitivity or excessive reaction of the immune system to a particular allergen.

An allergen is a substance that is foreign to the body and that can cause an allergic reaction in certain people. For example, pollen, dandruff, mold, some germs. In most people, the immune reaction to these foreign substances is normal and appropriate. But in allergic people, it is excessive.

For example, in people with hay fever, contact with pollen in the nose, throat and eyes triggers the mast cells to release much more histamine than normal.

This excessive release of histamine produces the associated symptoms of itching, swelling, watery eyes, etc. Antihistamines act by physically blocking the H1 receptors, preventing the histamine from reaching its target.

This decreases your body’s reaction to allergens and, therefore, helps reduce the annoying symptoms associated with allergy. Antihistamines are also used in the treatment of nausea and vomiting.

However, the exact way in which they relieve these symptoms is not fully understood.

The brain has several key areas that control vomiting. Antihistamines are thought to block H1 receptors in the brain area, which creates nausea in response to chemicals in the body.

Some antihistamines may also have what is known as an antimuscarinic effect. This means that the medicine can also block another type of receptor that is on the surface of certain cells.

If these receptors are affected, you may experience some of the side effects associated with antihistamines. For example, dry mouth, blurred vision and urine retention.

Note: antihistamines should not be confused with H2 blockers, which reduce the production of stomach acid. While both types of drugs block the actions of histamine, they work in different receptors in different systems of the body.

Types of antihistamines

Are there different types of antihistamines?

Generally, antihistamines have been classified into two groups:

  • First-generation antihistamines or sedatives: can cause significant drowsiness and are generally more associated with the anti-muscarinic side effects mentioned above.

These include Alimemazine, Chlorphenamine, Clemastine, Cyproheptadine, Hydroxyzine, Ketotifen and Promethazine. These medications can be used for their sedative effects if your sleep is disturbed by itching.

  • Non-sedating or second-generation antihistamines are newer medications that generally cause less drowsiness. However, anyone who takes these medications while performing specialized tasks, for example, driving, should know that there may still be a sedative effect and in particular, in combination with alcohol.

Second generation antihistamines include Acrivastine, Cetirizine, Desloratadine, Fexofenadine, Levocetirizine and Loratadine.

Which one is the best?

All antihistamines work quite well to reduce allergy symptoms. Your doctor or pharmacist can advise or prescribe a particular antihistamine based on the cause of your allergy and if you need a sedative or non-sedating medication. For example:

In general, antihistamines are probably equally effective in reducing the symptoms of hay fever (seasonal allergic rhinitis) and urticaria (urticaria).

However, non-sedating antihistamines tend to be used more frequently, since they cause less sleepiness.

Second generation antihistamines are generally recommended for most allergic situations, since they cause less drowsiness.

Cetirizine, Fexofenadine or Loratadine are often recommended for urticaria.

Antihistamine eye drops may be recommended when irritated eyes are a particular problem, for example, Azelastine drops or Ketotifen drops.

A sedating antihistamine can be particularly useful at bedtime for children who have allergic symptoms.

Cough medicines containing sedating antihistamines are not suitable for children under 6 years of age, and advice from the pharmacist is needed for children between the ages of 6 and 12 years. For other conditions, specific antihistamines may be used.

Chlorphenamine is the most used antihistamine in an emergency situation, such as anaphylaxis, and can be administered by injection in this situation. Diphenhydramine is sold without a prescription as a sleep aid.

What preparations are there?

Antihistamines come as:

  • Tablets (most commonly).
  • Liquid medications (for those who can not swallow tablets, for example, children).
  • Injections (for severe allergic reactions where immediate treatment is necessary). In drops for the eyes.
  • In drops for the nose and spray.
  • In creams and ointments.

Side effects and precautions

Can there be side effects?

Most people who take antihistamines do not have any serious side effects. If side effects occur, they are usually minor. The most common are:

  • Drowsiness
  • Headache
  • Dizziness
  • Agitation
  • Dry mouth
  • Blurry vision
  • Difficulty urinating (urinary retention)
  • Disorders of the stomach and intestine (gastrointestinal complaints).

For a complete list of all the side effects and possible interactions associated with your medication, see the booklet that comes with your medication.

Who should not take antihistamines?

Most people can take antihistamines safely.

Antihistamines should not be used by people with a rare metabolic disorder called acute porphyria. In addition, they may not be suitable for people with liver or kidney problems.

First generation antihistamines may not be suitable for men with enlarged prostate (benign prostatic hyperplasia).

They also may not be suitable for people with high pressure in the eye (acute glaucoma) or who are at risk of developing glaucoma. Women who are pregnant or breast-feeding should not usually take antihistamines.

This is because it is not known if they cause harm and you can not perform studies on women in this situation, just in case. However, it is not known that they cause harm.

If certain conditions, such as hay fever or morning sickness, make you very sick during pregnancy, the benefit of the treatment may be more than the minimum risk of any harm.

Your doctor will discuss this with you and prescribe one of the antihistamines that is considered safe during pregnancy if you choose to take one. Similarly, in breastfeeding women, in some cases the benefits may be more than the risk.

Antihistamines enter small amounts in breast milk, although they are not known to cause any harm. A complete list of people who should not take antihistamines is included with the information booklet that comes in the medication package.

If you are prescribed or buy an antihistamine, read this to make sure you can take it safely.


Can I drink alcohol when I’m taking an antihistamine?

Alcohol interacts with antihistamines. Probably the effect is greater for sedative antihistamines, but it can occur with any type. Alcohol and an antihistamine in combination are more likely to make you feel numb on your own.

Is it safe to take antihistamines with other medications?

Some medications may interact with antihistamines, increasing the side effects of one or both medications. It is usually best to avoid taking antihistamines at the same time as certain medications, including:

  • Certain types of antidepressants. Tricyclic antidepressants such as Amitriptyline and Lofepramine may interact with antihistamines. Other antidepressants such as Moclobemide.
  • Antacids, which can interfere with the absorption of antihistamine, which makes it less effective.
  •  Other tablets that cause drowsiness, since the combination may be excessive. For example, opioid analgesics or benzodiazepines.
  • A group of medicines called Antimuscarinics. This includes medicines such as hyoscine, oxybutynin, propantheline and tolterodine.

Because these medications can give you similar side effects, such as dry mouth or trouble urinating, the combination can aggravate side effects.

If you are buying your antihistamine at a pharmacy, be sure to tell the pharmacist if you take other medicines so they can advise you accordingly.

Also see the ingredients in other treatments without a prescription. For example, cough and cold remedies may contain antihistamines, in which case you may end up taking the double dose by mistake.