Antihistamines Include:

Ethanolamine- Carbinoxamine. Clemastine, Diphenhydramine.

Ethylenediamines- Pyrilamine, Triplennamine

Akylamines- Brompheneramine, Chlorpheniramine, Dexchlor pheniramine, Triprolidine Phenothiazines- Methidilazine, Promethizine

Piperadines- Azatadine, Cyproheptadine, Phenindamine

Miscellaneous- Astemazole, Loratadine, Terfenadine

Refer Chlorphenramine maleate

1. Inform physician of a history of glaucoma, peptic ulcer, urinary retension or pregnancy before starting antihistamine therapy.

2.May cause nervousness, insomnia and dry mouth.

3. May cause drowsiness or dizziness(except astemizole, loratadine and terfenadine). Patients should observe caution while driving or performing other tasks requiring alertness, coordination or physical dexterity.

4. Avoid alcohol and other CNS antidepressants.

5. May cause GI upset, take with food. Take Astemazole on an empty stomach, at least 2 hours after or 1 hour before a meal. Take Loratidine on an empty stomach

6. Avoid prolonged exposure to sunlight, may cause photosensitivity.

7. Astemizole/Terfenadine: patients should not take these agents if they have hepatic dysfunction or if they are also taking Ketoconazole, itraconazole or erythromycin

8. Phenothiazines: Patients should report any involuntary muscle movements or unusual sensitivity to sunlight

Pharmacology:

Antihistamines competitively antagonise histamine at the H1 receptor site, but do not bind with histamine to inactivate it. Terfenadine and Astemizole,,the most specific H1 antagonists available, bind preferentially to peripheral rather than central H1 receptors. Antihistamines do not block histamine release, antibody production or antigen-antibody interactions

Pharmacokinetics:

These agents are well absorbed following oral administration use, have an onset of action of 15 to 30 minutes., are maximal within 1 to 2 hours and have a duration of about 4 to 6 hours, although some are much longer acting

Terfenadine's- effects begin in 1 to 2 hours, reach maximum in in 3 to 4 hours and last in excess of 12 hours. It reaches peak in plasma levels in 2 hours and has an elimination half-life of 20 hours.

Astemizole- has a slow onset of action, and its effect last upto 24 hours based on once a day dosing. Its absorption is reduced by 60 %, if taken with food. Its half-life is biphasic; 20 hours for the distribution phase and 7 to 11 days for the elimination phase. Approximately 40 to 50% of the astimizole dose is excreted in the urine in 4 days, with 50 to 70% eliminated via the feces by 14 days.

Loratadine's- effects begin within 1 to 3 hours, reaching a maximum at 8 to 12 hours and lasting more than 24 hours. It is rapidly absorbed and extensively metabolised to an active metabolite. The mean elimination half-life is 8.4 hours for loratidine and 28 hours for its metabolite. Approximately 80% of the dose is equally distributed between the urine and feces.

Astemizole- Absorption by 60% if taken with food.

Pregnancy:

Chlorpheneramine, dextrochlorpheneramine, diphenhydramine, cyproheptedadine, clemastine, azatadine, methadilazine, loratidine, Astemizole, brompheneramine, promethazine, carbinoxamine, terrprolidine- Safety for use during pregnancy has not been established

Lactation:

Diphenhydramine, pyrilamine, tripelnnaime and lortadine and its metabolite pass esily into breast milk. Antihistamines are contraindicated to nursing mothers.