Antihistamines come as: An antihistamine tablet typically starts to work within 30 minutes after being taken.
The peak of effectiveness is typically within 1-2 hours after being taken.
While this is a helpful response, it also causes redness, swelling and itching.
Allergic reactions such as hay fever (seasonal allergic rhinitis) are caused by an oversensitivity or over-reaction of the immune system to a particular allergen.
Some antihistamines may also have what is known as an antimuscarinic effect.
This means that the medicine can also block another type of receptor found on the surface of certain cells.
An allergen is a substance that is foreign to the body and which can cause an allergic reaction in certain people. In most people, the immune reaction to these foreign substances is normal and appropriate. For example, in people with hay fever, contact with pollen in the nose, throat and eyes triggers the mast cells there to release much more histamine than normal.
Some of these medicines are available to buy over the counter from your pharmacist. These medicines come in a variety of forms, as mentioned above.
For example: For other conditions, specific antihistamines may be used.
For example, cyclizine and promethazine teoclate are used for feeling sick (nausea) and being sick (vomiting), not for hay fever.
Antihistamines are a group of medicines which act by blocking the action of the chemical called histamine in the body.
Either H1 or H2 histamine receptors can be blocked by medicines, but the group commonly known as antihistamines blocks the H1 receptor.
The brain has several key areas which control vomiting.