Hi, Antihistamines are medications that block the effects of histamine in the body. Histamine is a chemical released by the immune system in response to an allergen (such as pollen, dust, or animal dander) or other trigger. Antihistamines can be used to treat a variety of allergic symptoms, such as runny nose, sneezing, itchy eyes, and skin rashes. The duration of action of antihistamines depends on the type of medication. First-generation antihistamines, such as diphenhydramine (Benadryl), can cause drowsiness and last for about 4-6 hours. Second-generation antihistamines, such as cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra), are less likely to cause drowsiness and can last for 24 hours or longer. It is generally recommended to take antihistamines as directed on the label or as prescribed by a healthcare provider. Taking antihistamines twice a day is not uncommon in some cases, but it is important to consult with a healthcare provider before doing so, especially if the symptoms are not improving or if there are side effects such as drowsiness, headache, or chest discomfort. It is also important to identify and avoid triggers if possible, and to use other treatments as needed, such as nasal sprays, eye drops, or topical creams. The chest flutters described may or may not be related to the use of antihistamines, but it is important to report any new or concerning symptoms to a healthcare provider. Stress and other factors can also contribute to symptoms, and lifestyle changes such as exercise, relaxation techniques, and good sleep hygiene may be helpful in managing symptoms. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Priyanka, General & Family Physician