Do you also sneeze often? Do you have a runny nose when you wake up in the morning? Or do your eyes become red and itchy? Sometimes you suddenly start itching and scratching your body. These are the symptoms that we usually call "allergies." At times like these, we often go to the pharmacy and ask for an "allergy pill." That is the type of medicine we take called antihistamines. So today, let's talk about these medicines, how they work, their types, side effects, and things to be careful about while using them.
Simply put, what are these antihistamines?
Antihistamines are a class of medications, such as paracetamol and aspirin. They are mainly used to control symptoms related to allergies . They are most commonly prescribed for the things we talked about above, such as runny nose, sneezing, itchy eyes and skin.
But this type of medicine is not only used for allergies, but also for some other health conditions. For example, there are medicines in this category for some digestive problems, such as gastritis.
You can buy many antihistamines at a pharmacy without a prescription from a doctor. We call them (Over-the-counter - OTC) medications. Also, there are stronger antihistamines that a doctor prescribes. We can't just buy these at a pharmacy.
In addition, some of the medicines and tablets we take for the common cold and cough also contain antihistamines.
How does this medicine work inside the body?
To understand this, we first need to know what histamine is.
Think of histamine as a little messenger in our body. Our immune system produces this messenger. One of its main functions is to carry a message to other cells about something harmful or threatening to the body.
Now, let's look at how you get an allergy. When something you're allergic to (we call these allergens) enters your body, your immune system thinks, "Wow, there's a big enemy here!" Then the system goes into overdrive and releases a lot of these messengers called histamines all at once.
This histamine goes and sends messages to cells in different parts of the body.
- "The cells in the nose are told, 'Start making fluid quickly!'" That's when the fluid runs out of the nose.
- "It tells the cells in the skin, 'Start scratching!'" That's when the body starts scratching.
- "They tell the eyes, 'It's red, make tears!'" That's when the eyes get red and itchy.
Anti-histamineThe word itself means "against histamine." What this medicine does is block the message carried by that messenger called histamine from reaching the cells. It's like cutting off a call when it comes in. Then those allergic symptoms gradually decrease.
Simply put, histamine is the "cause" of an allergy. Antihistamines are medications that control the "symptoms" caused by that cause.
What are the main types of antihistamines?
Doctors divide antihistamines into two main types, based on the histamine receptors they block. Think of it like there are two types of antennas in our bodies that can pick up these histamine messages.
1. Antihistamines that act on H1 receptors
2. Antihistamines that work on H2 receptors
Now let's look at each of these types separately.
H1 antihistamines for allergies
These H1 receptors are spread throughout our bodies, especially in nerve cells, respiratory tracts, and blood vessels. In an allergy, histamine binds to these H1 receptors. That's when all the allergy symptoms we know come about. So, what H1 antihistamines do is block these H1 receptors.
The main uses of these H1 antihistamines are:
- Allergic rhinitis - We call it something like the common cold that comes with the winter season.
- Allergic eye disease (Allergic conjunctivitis)
- Skin allergies, eczema and rashes (Atopic dermatitis)
- Sinus infection (Sinusitis)
- Hives
- Motion sickness - This type of medicine is also given for vomiting and nausea while traveling on buses or vans.
- Insomnia - Sometimes doctors also recommend this type of medication to help you sleep.
These H1 antihistamines are further divided into two subgroups. This is very important, because it is good to know what type of medication you are using.
1. First-generation H1 antihistamines
These are the first antihistamines to be discovered. Their main characteristic is that they cause drowsiness. This is because these drugs can easily cross the blood-brain barrier that protects our brain and enter the brain. That is why we feel tired and sleepy when we take them.
- Examples: Chlorpheniramine - (the medicine known to many in Sri Lanka as 'Piriton' belongs to this category), Diphenhydramine, Hydroxyzine.
2. Second-generation H1 antihistamines
These are newly introduced and improved antihistamines. The biggest advantage of these is that drowsiness is greatly reduced or completely eliminated.These cannot easily cross the blood-brain barrier, so they are generally considered safer.
- Examples: Cetirizine, Loratadine, Fexofenadine, Desloratadine.
H2 antihistamines for stomach upset
H2 receptors are mainly found in the acid-secreting cells in our stomach. When stomach acid increases, conditions like gastritis, heartburn, and acid reflux occur.
H2 antihistamines block these H2 receptors and reduce the production of stomach acid.
The main uses of these are:
- Peptic ulcer disease
- Gastritis and GERD (chronic acid reflux)
- Zollinger-Ellison syndrome
- Examples: Famotidine, Cimetidine.
Be sure to keep these things in mind when using an antihistamine!
Although many antihistamines are available over the counter, we need to be very careful when using them. If used incorrectly, they can cause serious problems.
| What to do | What not to do (very important) |
|---|---|
| Follow your doctor's instructions exactly. Even if you buy a medicine over the counter from a pharmacy, read the instructions carefully. | Never double the prescribed dose. Don't think, "If I drink a little more, I'll feel better soon." |
| Know exactly when and how often to take your medication. Some medications are taken once a day, while others require multiple doses. | Do not take two types of antihistamines at the same time without the advice of a doctor. |
| If it is a first-generation medication that causes drowsiness, it is best to take it before going to bed at night . | Do not give children under 4 years of age over -the-counter cough and cold medications containing antihistamines without the advice of a doctor. |
| Inform your doctor about any other medications you are taking. | Do not drive or operate machinery while taking first-generation antihistamines that cause drowsiness. This can lead to very serious accidents. |
People over the age of 65 and young children are more sensitive to the effects of antihistamines. Therefore, be sure to consult a doctor before giving them this medication.
What are the side effects of antihistamines?
Like all medications, antihistamines can have side effects. However, not everyone gets them. These side effects vary depending on the type of medication.
| Type of antihistamine | Common possible side effects |
|---|---|
| H1 antihistamine (for allergies) | |
| First-generation (Types that cause drowsiness) |
|
| Second-generation (less sleepy types) |
|
| H2 antihistamines (for stomach upset) | |
| H2 antihistamine (e.g. Famotidine) |
|
Situations and risks that require special attention
If you have any of the following conditions, be sure to talk to your doctor before taking an antihistamine.
- Diabetes
- Enlarged prostate or difficulty urinating
- Epilepsy
- Heart disease
- High blood pressure
- Glaucoma (increased pressure in the eye)
- Hyperthyroidism
Overdose
Overdosing on antihistamines, especially the first-generation types that cause drowsiness, can be very dangerous. This can happen by mistake or when taken with other medications, such as sleeping pills.
Symptoms of an overdose:
- Excessive sleepiness
- Hallucinations
- Excessive restlessness and irritability in children
- Blurred vision
- Rapid heartbeat (Tachycardia)
- Seizures
If you or your child develops any of these symptoms, immediately take them to the nearest hospital Emergency Department (ETU). Or, call the National Poison Information Centre at the Colombo National Hospital .
Are these suitable for pregnant and breastfeeding mothers?
If you are pregnant, trying to become pregnant, or breastfeeding, it is essential to consult your doctor before taking any medication. Although studies have found that some antihistamines are safe during pregnancy, your doctor will decide what is best for you and your baby.
Take-Home Message
- Antihistamines are a major class of medications used to control allergic symptoms, and are also used to treat other conditions, such as gastritis.
- There are two main types: H1 (for allergies) and H2 (for stomach upset).
- The H1 type also comes in two parts: the first generation ones cause drowsiness , while the second generation ones have very little drowsiness.
- Never drive after taking antihistamines that cause drowsiness.
- Always follow the dosage and instructions given by your doctor or the package insert. Do not take too much.
- If you have other medical conditions, are pregnant, or are a nursing mother, be sure to consult a doctor before taking antihistamines.
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