Loratadine is a modern non-sedative antihistamine. It is most popularly known as Claritin and Alavert. Some users suffer adverse side effects from using Loratadine. Antihistamines are used to control the symptoms of allergies; they work by blocking histamine receptors and preventing the release of histamine. Histamine is the chemical released by the body when it reacts to a foreign substance. The release of Histamine causes a reaction which increases blood flow and releases other chemicals. (see References 1) Allergies are the body's overreaction to a harmless foreign substance.
Like Loratodine, Cetirizine (branded as Zyrtec and Reactine) very slightly crosses the blood brain barrier.(see References 2) This means that only a small amount of the drug enters the brain which eliminates the sedative effect common with older antihistamines. The older medications cross the blood brain barrier to a larger extent. Cetirizine can, however, still cause mild drowsiness.
Acrivastine is a new-generation antihistamine and thus crosses the blood brain barrier slightly and does not have a pronounced sedative affect. In the US, Acrivastine is the active ingredient in the Semprex and benadryl Allergy Relief capsules (see references 2). Acrivastine is the fastest acting of the modern antihistamines, however, it has to be taken three times a day.
Promethazine is an older sedative antihistamine. This type of antihistamine makes the user sleepy and should not be used when any kind of alertness is necessary. Although this type of medication causes drowsiness it is much more effective at treating the symptoms of eczema than the newer generation of antihistamines. (see References 1) Also Promethazine blocks histamine receptors in an area of the brain called the vomiting centre. This part of the brain is responsible for causing feelings of sickness and for the vomiting reflex. Thus Promethazine can help prevent and relieve motion sickness and travel sickness. Promethazine is the active ingredient in Phenergan.
Alimemazine is an older sedative antihistamine. This type of antihistamine makes the user sleepy and should not be used when any kind of alertness is necessary. Although Alimemazine causes drowsiness it is much more effective at treating the symptoms of eczema than the newer generation of antihistamines. Alimemazine is also used as a premedication to induce sleepiness in children before being given anaesthesia. Alimemazine is the active ingredient in Theralen and Nedeltran.