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 a 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 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)
  • I was feeling sick (nausea) and sick (vomiting).
  • Severe allergic reactions (anaphylaxis).

Where can I get them?

Some medications are available to buy without a 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 does 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. You can take the medication during the pollen season if you have hay fever.

How effective are they?

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, you may also need eye drops and a nasal spray to fix your symptoms completely.

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

In the summer, the pollen count is generally higher and may be in contact with the allergen regularly. 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 breastfeeding?

Pharmaceutical companies are reluctant to try medications on 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
  • I was 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 your body has not produced. (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 your immune system detects a foreign substance, histamine is released from the mast cells. Histamine binds to particular 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 helpful answer, it also causes redness, swelling, and itching. Allergic reactions such as hay fever (seasonal allergic rhinitis)

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

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

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

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 helps reduce the annoying symptoms associated with allergies. Antihistamines are also used in the treatment of nausea and vomiting.

However, the exact way 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 on the surface of specific cells.

If these receptors are affected, you may experience some side effects of antihistamines, such as 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 body systems.

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 aforementioned antimuscarinic side effects.

These include Alimemazine, Chlorphenamine, Clemastine, Cyproheptadine, Hydroxyzine, Ketotifen, and Promethazine. These medications can be used for 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 taking these medications while performing specialized tasks, for example, driving, should know that there may still be a sedative effect, particularly in combination with alcohol.

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

Which one is the best?

All antihistamines work pretty 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 hay fever symptoms (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 for children with allergic symptoms at bedtime.

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

Chlorphenamine is the most used antihistamine in an emergency, 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 severe 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 at risk of developing glaucoma. Women who are pregnant or breastfeeding should not usually take antihistamines.

This is because it is unknown if they cause harm, and you can not perform studies on women in this situation, just in case. However, it is not known whether 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 considered safe during pregnancy if you choose to take one. Similarly, in breastfeeding women, the benefits may be more than the risk in some cases.

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 in the medication package.

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


Can I drink alcohol when I am taking an antihistamine?

Alcohol interacts with antihistamines. Probably the effect is more significant 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 can interfere with the absorption of antihistamine, making it less effective.
  •  Other tablets that cause drowsiness since the combination may be excessive. For example, opioid analgesics or benzodiazepines.
  • A group of medicines is called Antimuscarinics. This includes medicines such as hyoscine, oxybutynin, propantheline, and tolterodine.

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

If you are buying your antihistamine at a pharmacy, 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, so that you may take the double dose by mistake.