They do not always completely control the allergic reaction because they do not counteract other chemicals that may be responsible for the symptoms.
There are at least two kinds of histamine receptors, hence are mainly H1 blockers.
That means you can control your symptoms with only 1 or 2 doses each day compared with older medications, which usually require doses every 4 to 6 hours to maintain their effectiveness.
The newer antihistamines are available only by prescription.
This means it has an effect on small blood vessels, resulting in dilated capillaries (redness), and leakage of protein-rich fluid into surrounding skin (swelling).
These effects of histamine cause sneezing, coughing, runny nose, and stopped up nose during colds.
They include the following: A common cold is a two step process (see How Cold Virus Infection Occurs and What Causes Cold Symptoms). The second step is the activation of the inflammatory mediators which directly cause the cold symptoms.
Ideally, it is desirable to treat both parts of the process but currently drugs for treating the virus infection (antivirals) are not commercially available.
As well as tablets, antihistamines are available as injections, elixirs, and creams.
An injection can be given in case of severe allergic reaction (although in such cases, adrenaline (epinephrine) may be more appropriate and can be life-saving).