First generation antihistamines include brompheniramine, diphenhydramine, carbinoxamine, chlorpheniramine, and clemastine. These antihistamines all have anticholinergic properties, making sedation one of the major side effects.Second generation antihistamines include cetirizine hydrochloride (Zyrtec), loratadine (Claritin, Claratyne), fexofenadine hydrochloride (Allegra, Telfast), and desloratadine (Claramax, Clarinex). Confusingly, these are sometimes also referred to as third generation antihistamines. They are all effective antihistamines that are less sedating and have relatively few side effects. Second generation antihistamines are less likely to cause impairment of cognitive facilities or performance skills. They have the added benefit that they usually only need to be taken one or twice daily to be effective. Cetirizine and loratadine are once daily antihistamines, while fexofenadine is taken once or twice daily, depending on its strength. When someone has an allergic reaction to an allergen, histamine is one of the substances released by the body in order to protect itself. In the case of an allergy, however, the release of histamine is actually unnecessary because the allergen is not the threat that the immune system mistakes it for. The mast cells that are spread throughout the tissues of the body release histamine as part of their defence mechanism against the foreign substance but the reaction usually remains localised. The histamine then binds to H1 receptors triggering the symptoms of an allergy as the cell becomes activated. Other chemicals are also released, adding to the reaction and prompting an increase in blood flow to the affected area. In the case of hayfever, these allergic symptoms include red, itchy, and watery eyes, sneezing, a runny nose, and nasal congestion caused by inflammation of the airways.Second generation antihistamines are non-sedating medications that work by preventing the actions of histamine. Second generation antihistamines do not actually prevent histamine from being released from the mast cells. They are selective peripheral histamine H1 receptor antagonists. This means that they prevent histamine from binding with H1 receptors, blocking the chain reaction that would normally lead to the physiological effects produced by histamine. These non-sedating second generation antihistamines are unlikely to cause drowsiness because they do not enter the brain in sufficient quantities. These second generation antihistamines are lipophobic and are unable to cross the blood to brain barrier when taken in normal dosages. Their large molecular size and the fact that they have a different ionic charge also impedes this transfer. In some cases, however, individuals with a low threshold for the medication may find they experience slight sleepiness after taking them or after taking a higher than recommended dose.Second generation antihistamines are effective in treating both seasonal allergic rhinitis (hayfever) and perennial allergic rhinitis. They are also particularly useful for treating a condition known as chronic idiopathic urticaria. This manifests itself as a chronic itchy rash that has no apparent cause. Second generation antihistamines reduce the rash and relieve the itching of chronic idiopathic urticaria.Each of the second generation antihistamines may be more suited to treating some conditions rather than others. For example, cetirizine can be more effective at relieving allergy symptoms, especially controlling itching and swelling. As a piperazine derivative, however, it is also the more likely to cause drowsiness when used in higher doses than the other second generation antihistamines. Fexofenadine, on the other hand, is the least sedating of any of the second generation antihistamines, even at higher dosages. Loratadine, however, may be better at improving allergy symptoms over a short period of time than fexofenadine. Loratadine and cetirizine are both available in syrup form making them the most suitable for use by younger children. Desloratadine is a purified single isomer version of loratadine, which contains a mixture of active and inactive isomers. In this way desloratadine is designed to have even less side effects than loratadine because it only uses the therapeutically-active isomer.