First-generation antihistamines

First-generation antihistamines competitively and reversibly block histamine H1 receptors, so histamine released in allergy can no longer make small vessels leak and swell, cause itch or contract smooth muscle. They enter the brain readily and block its H1 receptors, which causes sedation, and they also block muscarinic receptors, giving anticholinergic effects such as dry mouth and blurred vision.

• ChlorphenamineChildren and older people are the most prone to its anticholinergic effects and to paradoxical excitement.
• PromethazineIts weak dopamine block, about a tenth of chlorpromazine's, comes with rare extrapyramidal effects and NMS, as with other phenothiazines.

• Hay fever and allergic rhinitis
• Hives and itchy allergic skin
• Allergic reactions, as an add-on after epinephrine
• Nausea, vomiting and motion sickness (promethazine)
• Short-term sedation (promethazine)

• Drowsiness and sedationKnown for
• Dry mouth, blurred vision, constipationKnown for
• Difficulty passing urine
• Dizziness and poor coordination
• Restlessness and excitement in children
• Confusion in older people
• Headache
• Nausea
• Blood disorders such as agranulocytosisSerious
• Breathing suppressed in young children (promethazine)Serious

What the class has in common. Each drug's card has its own, and Known for marks what an exam expects you to know.

• Sedation can impair driving, and alcohol, sedatives and opioids add to it.
• Anticholinergic effects call for care in glaucoma, prostate enlargement and older people.
• MAO inhibitors intensify their effects; they are not given within 14 days of one.
• Promethazine can cause fatal respiratory depression in young children: not under 2 years (US) or 6 years (UK).

Written for biofideX in plain words, after checking these references:

• Chlorpheniramine maleate tablets: label (DailyMed)
• Promethazine hydrochloride tablets: label (DailyMed)
• Chlorphenamine allergy tablets: SmPC (emc)
• Promethazine hydrochloride (NCI Thesaurus)
• Chlorphenamine (NHS)

For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.