Antihistamines block the effects of histamine in the body. Histamine is released from the cells when the body has an allergic reaction, when cells are injured, or in reaction to a variety of drugs and poisons. Common antihistamines include promethazine, dimenhydrinate, diphenhydramine, tripelennamine and cimetidene.
None in common use.
Forms and Appearance
Most antihistamines are white, odourless, crystalline powders which have a bitter taste. Tablets and liquids vary in colour.
Medical and Other Uses
Antihistamines are used to treat allergic reactions such as hay fever; to dry the runny nose from a cold; to cause drowsiness and sedation; to prevent travel sickness and the nausea and vomiting associated with radiation therapy.
Street drug combinations include ‘blue velvet’ (paregoric and the antihistamine tripelennamine) and ‘T’s and Blues’ (pentazocine and tripelennamine). (Paregoric is a solution containing opium; pentazocine is a pain reliever.)
Methods of Use
Most antihistamines are administered orally, by tablet, capsule or liquid. However, injections may be used in the emergency treatment of severe allergic reactions. The street drug combinations, (given above under ‘Combination products’), are usually injected intravenously.
Effects of Use
Antihistamines taken alone produce only mild euphoria. However, it appears that some antihistamines increase the pleasurable effects of alcohol and other drugs which depress the Central Nervous System. The following effects can vary between different antihistamines:
Low doses can cause:
- poor concentration
- mild incoordination
- sometimes insomnia
- excitation and nervousness (most commonly in children)
- blurred vision
- ringing in the ears
- infrequently tremors
- reduced blood pressure
- dry nose and throat
- dry mouth
- stomach discomfort
- loss of appetite
- relief from nausea OR increased nausea and vomiting
- constipation or diarrhoea
- frequent urination
- heaviness and weakness in the hands
- muscle pain
- tightness in the chest
Higher doses intensify the effects described above. Sleep is the most likely result of a high dose, after drowsiness, lethargy and sedation.
Tolerance and Dependence
The effectiveness of antihistamines tends to diminish with long-term regular use. Little is known about the effects of long-term high dose abuse.
There is not enough evidence at the moment to say whether long-term regular use results in physical dependence. However, regular users may become psychologically dependent.
Very high doses in children can be as few as 15-20 tablets of most over-the-counter antihistamines.
The effects of these doses can be extremely severe, including:
- fixed and dilated pupils
- muscle uncoordination
- very high fever
These symptoms may be followed by coma, profound cardiovascular and respiratory depression, and sometimes death.
The effects of a non-fatal overdose on adults can include:
- toxic psychosis with hallucinations
- delusions and disorientation
- muscle twitching
- irregular heartbeat
- abnormally high blood pressure
- convulsions (rarely)