Antihistamines are drugs used to treat the symptoms of allergic reactions. They can be taken in cases of:


  • Allergy to dust mites
  • Seasonal allergy (the so-called hay fever)
  • Food Allergies


However, they are not able to control all the possible symptoms of the allergic reaction. In case of nasal congestion, for example, a decongestant might be more appropriate, so much so that some medicines combine the use of an antihistamine to an active substance of this type.


The mechanism of action of these drugs is based on the block of histamine, the substance released by immune cells when the body comes in contact with the allergen in respect of which it is hypersensitive. It is histamine, acting as a mediator of inflammation, to trigger the typical symptoms of allergic reactions, from swelling of the tissues of the nose, increased tearing of the eyes, to itching and in some cases even rashes.


How should antihistamines be taken?


Antihistamines are available for use in the form of tablets, capsules, solutions, nasal sprays, eye drops and creams. In some cases, these are over-the-counter medications that can be obtained without a medical prescription. However, it is always recommended to seek advice from your doctor or pharmacist if you think you have to deal with the symptoms of an allergy, in order to purchase the right product for your case.


Typically, an antihistamine effect begins 15- 30 minutes after intake, and reaches the maximum effectiveness in 1 to 2 hours. The antihistamines of the new generation have a prolonged effect that can reach 24 hours and thus it can also be taken only once a day.


Contraindications and warnings associated with the use of antihistamines


Antihistamines may have different adverse effects. Those commonly referred to as “first-generation“ antihistamines tend to trigger more side effects, while those known as "new generation" antihistamines have fewer adverse effects. In general, when taking an antihistamine the following side effects may occur:


  • Dry mouth
  • Drowsiness
  • Dizziness
  • Nausea and vomiting
  • Problems with urination
  • Blurred vision
  • Sense of confusion
  • In children, restlessness, or mood swings


In general, it is not recommended to take antihistamines before driving or before using dangerous machinery. Rather, if the drug causes drowsiness it is good to take it at bedtime.


It is also recommended to consult your doctor if you want to take an antihistamine in the following conditions:


  • You are pregnant or you are breast-feeding
  • You suffer from an enlargement of the prostate
  • You suffer from high blood pressure or cardiovascular disorders
  • You have problems with the thyroid gland, kidneys or liver
  • You have a condition associated with an obstruction of the bladder
  • You suffer from glaucoma