Why are antihistamines (antihistamine medications) taken on an empty stomach?

From the Guidelines

Antihistamines should be taken on an empty stomach to maximize their effectiveness, as food can interfere with their absorption and reduce their efficacy. According to the clinical guideline for the evaluation and management of chronic insomnia in adults 1, administration of sedatives/hypnotics, including antihistamines, on an empty stomach is advised to maximize effectiveness. This is because food can slow down the absorption of antihistamines or bind to the medication, potentially reducing its effectiveness.

Some key points to consider when taking antihistamines include:

• Taking antihistamines about one hour before or two hours after meals allows for faster and more complete absorption into the bloodstream
• First-generation antihistamines like diphenhydramine may cause stomach upset and might be better tolerated with a small amount of food
• Second-generation antihistamines like cetirizine and loratadine are generally less affected by food
• If you experience stomach discomfort when taking antihistamines on an empty stomach, it's acceptable to take them with a light snack

It's essential to follow the specific instructions on your medication label or from your healthcare provider, as recommendations can vary based on the specific antihistamine formulation and your individual health needs 1. Additionally, certain antidepressants and over-the-counter sleep medications containing antihistamines may have different recommendations, and their efficacy for treating insomnia is not well established 1.

From the Research

Antihistamine Administration

• Antihistamines are typically taken on an empty stomach to facilitate faster absorption and onset of action 2.
• The efficacy of antihistamines can be influenced by their administration, with some studies suggesting that taking them on an empty stomach can enhance their effectiveness 2.

Mechanism of Action

• Antihistamines work by inhibiting histaminic actions, primarily through competition with histamine for histamine receptors 3.
• They can be classified into different types, including H1-receptor inhibitors and H2-receptor inhibitors, each with distinct therapeutic applications 3.

Pharmacological Activities

• Antihistamines can exert a wide range of pharmacological activities, including CNS depression, dryness of mucous membranes, and gastrointestinal effects 3.
• Their use can be accompanied by undesirable side-effects, highlighting the need for judicious administration and proper dosage 3.

Guidelines for Use

• Antihistamines can be used to maximum benefit if guidelines are followed, including around-the-clock administration, titrated dosage, and substitution to a different class of antihistamine if necessary 4.
• The efficacy of antihistamines can diminish with prolonged use, either due to true tolerance or psychic factors 4.

Therapeutic Uses

• Antihistamines have a range of therapeutic uses, including the treatment of allergic disorders, allergic rhinitis, and urticaria 5.
• Newer antihistamines, such as loratadine and desloratadine, are potentially safer and cause less sedation than first-generation antihistamines 6.