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., immunology experts have concluded there is now “little role for sedating antihistamines in allergic conditions”.While early ‘first generation’ antihistamines such as promethazine cause sedation, this is less of a problem with newer ‘second generation’ antihistamines such as loratadine, and ‘third generation’ ones such as desloratadine.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.
In this way desloratadine is designed to have even less side effects than loratadine because it only uses the therapeutically-active isomer.
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.
The newer antihistamines are likely to be as safe in pregnancy but have not been used by as many women, so they do not have the same evidence of safety.”Cetirizine is one of the newer antihistamines most likely to cause sedation, they say, particularly in higher doses.
Meanwhile, the authors remind readers that there is no role for antihistamines for cold and flu symptoms, nor for acute management of anaphylaxis.
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.