Allergies and Drugs: What's the Difference Between First-Generation and Second-Generation Antihistamines?

By Cristiano Antonino On Mar 31, 2022

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Antihistamines are a class of drugs that help to treat the symptoms of various allergies. The main difference between first-generation and second-generation antihistamines is that the latter do not cause sleepiness and are considered safer because they interact better with other medications

What Are Antihistamines Used For?

Antihistamines are medications that block chemicals in the body called histamines.

Histamines are the chemical that gets released after you are exposed to something you are allergic to.

They cause the symptoms of an allergic reaction, like:

Itchiness
Hives
Runny nose
Itchy eyes
Sneezing
Insomnia
Nausea
Vomiting
Fatigue

There are two different types of histamines: H-1 receptor antagonists and H-2 receptor antagonists

Typically, antihistamines that treat H-2 receptor antagonists treat gastrointestinal symptoms.

Both first-generation and second-generation antihistamines treat H-1 receptor antagonists.

H-1 receptor antagonists treat:

Colds
Food allergies
Hives
Hay fever
Insect bites
Reactions to medications

What Are First-Generation Antihistamines?

First-generation antihistamines became available for general use in 1942 and are still used today.

These antihistamines work by affecting the histamine receptors in the brain and spinal cord.

One of their defining characteristics, though, is that they go through the blood-brain barrier and can cause sleepiness.

Some examples of first-generation antihistamines include:

NyQuil
Tylenol Cold and Cough Nighttime
Periactin
Dayhist
Chlor-Trimeton

These antihistamines start to take effect in about 30 to 60 minutes and last for four to six hours.

The most popular first-generation antihistamine is chlorpheniramine, especially for emergency use.

Chlorpheniramine can be found in the following over-the-counter antihistamines:

Advil
Tylenol
Chor-Trimeton
Dimetapp

Some of the side effects of first-generation antihistamines, besides drowsiness, are:

Dry mouth and eyes
Blurry vision
Headaches
Dizziness
Lowered blood pressure
Mucous thickening
Increased heart rate
Constipation
Trouble urinating

What Are Second-Generation Antihistamines?

Second-generation antihistamines were first developed in the 1980s.

They cause less sleepiness than first-generation antihistamines and also interact with fewer medications.

Some examples of second-generation antihistamines include:

Claritin
Zyrtec
Allegra
Clarinex

You can take second-generation antihistamines orally, nasally, or via eye dropper.

They typically last for up to 24 hours.

They are able to reduce the inflammation caused by allergies and are favored both because they have fewer side effects and can be more effective in treating allergy symptoms.

Some side effects of second-generation antihistamines are:

Headache
Cough
Fatigue
Sore throat
Stomach pain
Nausea
Vomiting

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Which Type of Antihistamine Should I Take?

There are many different types of antihistamines to take, most of which can be prescribed or bought over-the-counter.

Due to the sheer volume and the various ways that these medications treat different symptoms, you may need the guidance of a doctor or pharmacist.

However, for less severe allergies, you can probably take over-the-counter antihistamines.

If you have more severe symptoms, you may need to be prescribed antihistamines by your doctor.

If this happens, you and your doctor will need to work closely together.

Children, pregnant women, and older people are more sensitive to antihistamines.

Follow the dosage instructions on the medicine package.

It is important that you do not take more than one antihistamine at a time unless otherwise instructed by your doctor.

You may need to take more than one antihistamine in order to figure out which ones work for you, but you should always try new medications at different times.

You should also pay special attention to medications that the antihistamines may interact with.

If you are taking quite a few medicines, chances are that you should probably take second-generation antihistamines.

If antihistamines do not work for you, you should speak to your doctor about an alternative, such as a decongestant.

You should not take any sort of first-generation antihistamine if you have any of the following conditions:

Glaucoma
Difficulty with urination
Asthma
Emphysema
Chronic bronchitis
Thyroid disease
Heart disease
High blood pressure

You should keep your antihistamines in a cold, dry place out of reach from children.

It is not recommended that you store antihistamines in bathrooms, as bathrooms can get hot and humid. In these environments, antihistamines can lose their effectiveness.