Any of various compounds that counteract histamine in the body and that are used for treating allergic reactions, hay fever and cold symptoms.

There are a couple of kinds of antihistamines now, the old kind and the new and improved kind. The new kind only release enough histamine blocking drug prevent you from sneezing and wheezing (or whatever else you allergy sufferers do). The old kind, which are much more fun if you ask me, release enough histamine blocking drug to permeate every single cell in your body.

I can hear you now saying, isn't that better? Doesn't more drug mean less sniffles? Well, yeah, but your brain relies on histamine to function properly, so the old kind of pills will thoroughly zonk you. If you're driving, take the new ones. If you're looking for a little more entertainment during the allergy season and you're not an air traffic controller or anything, then give the old ones a try.

Brand names of the new kind: Claritin-D, Allegra, etc.
Brand names of the old kind: Whatever's at Wallgreen's.

In addition, if your antihistamine happens to say something like avoid alcohol, it's a very good idea to heed that warning.

If you ignore it (as I once - accidentally - did) there's every chance that you will pass out, start vomiting, and probably not remember anything the morning after.

As T.V. adverts always say: Always Read The Label :)


Brand/Generic Drug Names

brompheniramine, budesonide, cetirizine, chlorpheniramine, cyproheptadine, diphenhydramine, Fexofenadine, promethazine, triprolidine
Common uses
control symptoms of allergies, rhinitis, pruritus
compete with histamine for H1-receptor sites
Class contraindications
hypersensitivity, acute asthma, lower respiratory disease, narrow-angle glaucoma, bladder neck obstruction, stenosing peptic ulcer, symptomatic prostatic hypertrophy, newborn, lactation
Class precautions
increased intraocula pressure, renal and cardiac disease, hypertension, seizure disorders, pregnancy, lactation, elderly
increased central nervous system depression: barbiturates, narcotics, hypnotics, tricyclic antidepressants, alcohol
Adverse Reactions
drowsiness, headache, thickening of bronchial secretions, blood dyscrasias, urinary retention, gastrointestinal problems
Additional Information
Assess intake and output, cardiac and respiratory status
Assess complete blood count, blood dyscrasias
Provide hard candy for dry mouth
Evaluate therapeutic response: absence of allergic symptoms and pruritus
Date of most recent Update
August 07, 2002
Further information is available in the writeup for the specific name(s) of this medication class

Before delving too far into antihistamines, one should be aware of what a histamine is for an antihistamine to be anti- of. A histamine is a compound that is part of many venoms and stings, it is also produced naturally by the immune system as a response to tissue damage. When histamines are released they produce an immediate allergic response that often includes inflammation of the tissue and a host of other actions (such as increased gastric acid release). The release of a histamine also changes the size of blood vessels (known as vasodilation) that allows the cells that handle whatever happened to get there faster.

Inflammation caused by histamines happens when the histamine binds to a receptor on a smooth muscle cell that lines blood vessels causing them to dilate. However, in the digestive system and lungs the histamine causes the smooth muscles to contract (this is why allergic reactions make it hard to breath).

An antihistamine is a drug that blocks the receptor on the smooth muscle known as H1 for histamine, type 1. There are drugs that also block the H2 receptors - these are often used to treat ulcers by stopping the gastric acid release (for example, the drug Tagamet). The antihistamine 'competes' with the histamines for receptors and thus reduces the level of an allergic reaction. When it comes down to numbers, the antihistamines win - outnumbering the histamines by several orders of magnitude.

Antihistamines are often taken in the form of a pill which is absorbed in the gut and metabolized in the liver. This liver action is part of the reason for the warning of don't consume alcohol - which is also metabolized in the liver. This may cause alcohol poisoning because the alcohol didn't get metabolized from a poison to something less deadly soon enough.

The dopy nature of antihistamines that comes from the fact that histamines are also neurotransmitters. In this case, the histamine keeps the brain awake and alert. Newer antihistamines are targeted to the body but not the brain (this actually is not that difficult - most medications don't get to the brain).

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