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How long do antihistamines take to work?

Medically reviewed by Sally Chao, MD. Last updated on Sep 23, 2024.

Official answer

by Drugs.com

In general, an oral antihistamine tablet will start to work within 30 minutes and reach its maximum effect in about 2 hours. You will know when an antihistamine starts to work when it starts to relieve allergy symptoms, such as congestion, runny nose, itchy eyes or skin rash.

Antihistamines can be over-the-counter or prescription medications. Prescription medications include antihistamines that are injected. Antihistamines can also be used in eye drops, creams and nasal sprays. In these cases, the amount of time the antihistamine takes to work may be different.

Depending on the type of antihistamine you take, there may be side effects that you notice when an antihistamine starts to work. These include:

  • Drowsiness
  • Dizziness
  • Decreased appetite
  • Blurred vision
  • Dry mouth

Some people may feel excited or anxious.

Antihistamines are often used to treat allergic symptoms. They may also be combined with other medications and ingredients in over-the-counter cough and cold medication.

Older first-generation antihistamines can cross into the brain and cause a type of drowsy feeling called sedation. These antihistamines may be used to treat insomnia, anxiety, nausea and motion sickness.

Second-generation antihistamines are less likely to cause drowsiness, and their effects last longer. They are best used for long-term allergy conditions.

References
  1. Health Navigator New Zealand. Antihistamines. August 2021. Available at: https://www.healthnavigator.org.nz/medicines/a/antihistamines/. [Accessed September 4, 2021].
  2. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet]. Bethesda (MD): National Institute of Diabetes and Digestive and Kidney Diseases; 2012-. Antihistamines. [Updated 2017 Jan 16]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK547896/.
  3. U.S. National Library of Medicine MedlinePlus. Antihistamines for allergies. May 2020. Available at: https://medlineplus.gov/ency/patientinstructions/000549.htm. [Accessed September 4, 2021].

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