Antihistamines are popularly used to treat symptoms caused by allergies, such as sneezing, runny nose, and itchy eyes. Available in different forms, these medications continue to help individuals struggling with various conditions. Certain generations of the drug are known to cause drowsiness and this has affected its use in certain circumstances. Here are answers to the question which antihistamines make you drowsy? This will help users get a fair idea of the most suitable antihistamine for use without any undesirable outcomes.

What are antihistamines?

As outlined above, antihistamines are a class of drugs that treat symptoms of allergies. The medication works by blocking the effects of histamine, a chemical released by the body during an allergic reaction. Antihistamines are available as over-the-counter products and also as prescription drugs. Common over-the-counter antihistamines include diphenhydramine, loratadine, and cetirizine. Prescription antihistamines include desloratadine and levocetirizine. Antihistamines are available in various forms including tablets, capsules, syrups and eye drops.

What is the mechanism of action of antihistamines?

The mechanism of action of antihistamines is blockage of the effects of histamine, a chemical released during an allergic reaction. Histamine acts on receptors in the body called H1 receptors, which are found in various tissues such as the nose, eyes, lungs, skin, and gut. When histamine binds to these receptors, it causes a wide range of symptoms such as sneezing, runny nose, itchy eyes, and increased mucus production.

Antihistamines competitively bind to these H1 receptors and prevent histamine from binding to them. By doing so, antihistamines reduce or eliminate the symptoms caused by histamine, such as itching, sneezing, and runny nose. Some antihistamines, such as diphenhydramine, also have sedative properties and this is one of the reasons for the drowsiness as a side effect.

It is also important to note that there are also H2 receptors, found in the stomach, and H3 receptors, found in the brain; however, antihistamines work by mainly targeting H1 receptors.

What are the conditions treated by antihistamines?

Antihistamines are primarily used to treat allergic symptoms, such as hay fever, allergic rhinitis, hives, and allergic conjunctivitis. These conditions are caused due to response to an allergen, such as pollen, dust mites, or animal dander. By blocking the effects of histamine, antihistamines help reduce/eliminate sneezing, runny nose, itchy eyes, and skin rash. In addition to treating allergic symptoms, antihistamines can also be used to treat other conditions such as:

Note of caution about conditions that are not to be treated with antihistamines

Antihistamines are not to be used to treat an asthma attack, and they should also be used with caution in patients with narrow-angle glaucoma, urinary retention, or prostate enlargement. It is necessary to seek medical advice or consent prior to using antihistamines for any new condition. This applies to over the counter formulations as well as prescription drugs that belong to the category of antihistamines.

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What are the different generations of antihistamines?

As mentioned above, there are different generations of antihistamines; and these refer to the various groups of drugs that have been developed over time to treat allergic symptoms. Each generation has its own unique properties and side effects, and they differ in their chemical structure, potency and duration of action.

1st generation antihistamines: These are the first antihistamines to be discovered, and they are also known as “sedating antihistamines” because they have a strong sedative effect. They are also referred to as “older antihistamines” and include drugs such as diphenhydramine, chlorpheniramine, and brompheniramine. They have a high affinity for H1 receptors and tend to be more lipophilic, which means they can easily cross the blood-brain barrier and affect the central nervous system. Due to this, they have a higher risk of causing drowsiness, dizziness, and cognitive impairment. They also have a longer duration of action, which can last up to 8 hours.

2nd generation antihistamines: These are also known as “non-sedating antihistamines” because they have less of a sedative effect than first-generation antihistamines. They are also referred to as “newer antihistamines” and include drugs such as loratadine, cetirizine, and fexofenadine. They have a lower affinity for H1 receptors and tend to be more hydrophilic, which means they do not easily cross the blood-brain barrier and have less of an effect on the central nervous system. Due to this, they have a lower risk of causing drowsiness, dizziness, and cognitive impairment. They also have a shorter duration of action, which can last up to 12 hours.

3rd generation antihistamines: These are also known as “ultra-non-sedating antihistamines” and include drugs such as desloratadine and levocetirizine. They are long-acting and selective H1 receptor antagonists, which means they have a high specificity for H1 receptors, and they have a lower risk of side-effects. They have a lower risk of causing drowsiness, dizziness, and cognitive impairment. They also have a longer duration of action, which can last up to 24 hours.

The above classification of antihistamines into generations is not absolute and it is possible that there may be some overlap in the properties of drugs within each generation. Additionally, it’s important to bear in mind that all individuals are unlikely to experience the same side effects from the same antihistamine, and certain antihistamines may cause drowsiness in some people while others do not. The best method is to seek medical advice to identify antihistamines that may be most suitable.

Antihistamines that are generally known to cause drowsiness

Some common antihistamines that can cause drowsiness include diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), cetirizine (Zyrtec), and loratadine (Claritin). However, as outlined above, all individuals may not experience drowsiness from these medications. It is also important to note that certain formulations classified as not causing drowsiness, may also figure on the list of antihistamines that cause drowsiness. This is because of the formulation or the combination formulation.

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