Antihistamines

Question 1

MoA

Answer 1

Binds to H1 receptor site.
Blocks the action of histamine

Question 2

Old generation antihistamines (sedating)

Answer 2

Promethazine
Chlorphenamine
Hydroxyzine

Question 3

New antihistamines (non-sedating)

Answer 3

Acrivastine
Cetirizine/levocetirizine
Loratadine

Question 4

Promethazine

Answer 4

Most sedating
BD/TDS

Question 5

Chlorphenamine

Answer 5

QDS

Question 6

Hydroxyzine

Answer 6

QT prolongation + torsades de pointes
Use = short period only, max dose = 100 mg

Question 7

Acrivastine

Answer 7

TDS

Question 8

Cetirizine/levocetirizine

Answer 8

OD

Question 9

Loratidine

Answer 9

OD

Question 10

Fexofenadine

Answer 10

OD

Question 11

Other uses

Answer 11

N + V
Insomnia
Adjunct in emergency anaphylaxis and angiodema

Question 12

N + V

Answer 12

Cinnarazine
Cyclizine
Promethazine hydrochloride
Promethazine teoclate
Buclizine (migraines only, sold OTC)

Question 13

Insomnia

Answer 13

Both sold OTC
- Promethazine hydrochloride
- Diphenhydramine

Question 14

Adjunct in emergency anaphylaxis and angioedema

Answer 14

Chlorphenamine
Promethazine injection

Question 15

Side effects

Answer 15

Old antihistamines = more sedating + antimuscarinic
New antihistamines = less sedation + less psychomotor impairment

Question 16

Alcohol

Answer 16

Avoid alcohol
Driving may be imapired

Question 17

Cautions

Answer 17

• BPH (urinary retention)
• Glaucoma (raised intraocular pressure)
  Severe liver impairment (sedation precipitates hepatic coma)

Question 18

Risk factors for QT prolongation (hydroxyzine)

Answer 18

• Concomitant drugs that prolong QT interval
• CV disease
• FH of sudden death
• Hypokalaemia
• Hypomagnesaemia
• Bradycardia