Phenergan

What Is Phenergan and How Does It Work?

Phenergan is the brand name for promethazine hydrochloride, a first-generation antihistamine that also has sedative, antiemetic (anti-nausea), and anticholinergic properties. It belongs to the phenothiazine class of antihistamines and works primarily by blocking histamine H1 receptors, thereby preventing the effects of histamine, the chemical responsible for allergy symptoms such as itching, sneezing, runny nose, and urticaria (hives). Unlike modern non-sedating antihistamines such as cetirizine or loratadine, promethazine readily crosses the blood-brain barrier, which accounts for its significant sedative effects.

Phenergan is used to treat a range of conditions. In allergy management, it relieves symptoms of hay fever, perennial allergic rhinitis, urticaria, and other hypersensitivity reactions. Its antiemetic properties make it effective in the prevention and treatment of nausea and vomiting associated with travel sickness and post-operative nausea. The sedative effect of promethazine is exploited therapeutically for short-term management of insomnia and as a pre-operative sedative. It is also used alongside analgesics in some pain management regimens, as it can potentiate the effect of opioids and reduce nausea associated with opioid use.

Sedation and Nighttime Use

The sedative properties of Phenergan are one of its most pronounced characteristics. A single 25 mg dose typically produces drowsiness within thirty minutes to an hour, with effects lasting up to twelve hours. This makes it suitable for use at bedtime for patients struggling with short-term insomnia, but equally means that it should not be taken before driving, operating machinery, or undertaking tasks requiring concentration.

Formulations Available

Phenergan is available as 10 mg and 25 mg tablets, as well as a 5 mg/5 ml oral elixir (syrup) for those who cannot swallow tablets. It is also available as an injection for hospital use. The oral preparations are available without a prescription from pharmacies for adults and children over 10 years of age for specified indications, though prescription strength and wider indications require a doctor's prescription.

How to Take Phenergan

Phenergan tablets should be swallowed whole with water and can be taken with or without food. To minimise daytime sedation when using Phenergan for allergies, the 25 mg dose is typically taken at night, while for motion sickness it should be taken 30-60 minutes before travel commences. If daytime dosing is needed for allergy symptoms, doses may be split throughout the day, though drowsiness should be anticipated.

When using Phenergan for insomnia, a single 25 mg dose should be taken at bedtime. Do not take more than the recommended dose, as this increases the risk of side effects including excessive sedation, confusion, and anticholinergic effects. Phenergan is not intended for prolonged daily use as a sleep aid, if insomnia persists beyond a few days, speak to your GP about the underlying cause and appropriate management.

Motion Sickness Prevention

For travel sickness in adults and children over 10, Phenergan (25 mg) should be taken the night before travel and repeated the following morning if a long journey is anticipated. For children between 5 and 10 years of age (on prescription), the dose is 10-12.5 mg. Phenergan should never be given to children under 2 years of age due to the risk of fatal respiratory depression.

Phenergan (promethazine hydrochloride) is available in 10 mg and 25 mg tablets and a 5 mg/5 ml elixir.

Allergic conditions (adults): 10-20 mg two to three times daily, or 25 mg at night. Insomnia (adults): 25 mg at bedtime. Maximum 50 mg per night. Motion sickness (adults): 25 mg taken 30-60 minutes before travel; repeat after 6-8 hours if needed. Children (5-10 years, prescription only): 5-12.5 mg twice daily or at night. Children (10 years and over): adult dose may be used under supervision.

Phenergan should be used with caution in elderly patients, who are more sensitive to sedative and anticholinergic effects, and at lower doses where possible. Reduce dose in patients with renal or hepatic impairment. Promethazine is not recommended in patients with epilepsy, as it may lower the seizure threshold.

Common Side Effects

• Drowsiness and sedation (most common)
• Dry mouth
• Blurred vision
• Urinary retention or difficulty passing urine
• Constipation
• Dizziness or lightheadedness
• Restlessness or agitation (paradoxical reaction, more common in children)
• Headache

Serious Side Effects

• Respiratory depression: particularly dangerous in children under 2, do not use in this age group
• Neuroleptic malignant syndrome: high fever, muscle rigidity, altered consciousness (rare)
• Extrapyramidal reactions: involuntary movements, tremor, muscle stiffness (more likely at high doses)
• Severe allergic reactions: anaphylaxis, angioedema
• QT prolongation: abnormal heart rhythm, especially with other QT-prolonging drugs

Critical Warning: Children Under 2 Years

Phenergan must never be given to children under 2 years of age. Cases of fatal respiratory depression have been reported in young children given promethazine. The UK Medicines and Healthcare products Regulatory Agency (MHRA) has issued strong guidance that promethazine-containing products are contraindicated in this age group. In children aged 2-5, promethazine should only be used under direct medical supervision, not as an over-the-counter purchase.

Phenergan causes significant drowsiness and impairs psychomotor function. Patients must not drive, cycle, or operate heavy machinery while taking Phenergan, and should avoid alcohol, which markedly enhances the sedative effect. Elderly patients are particularly susceptible to anticholinergic side effects (confusion, urinary retention, constipation) and falls due to dizziness, and should use Phenergan with caution.

Precautions for Specific Medical Conditions

Phenergan should be used with caution in patients with glaucoma, urinary retention, prostatic hypertrophy, epilepsy, hepatic impairment, or a history of cardiac arrhythmias. Concomitant use with other CNS depressants (opioids, benzodiazepines, antidepressants, alcohol) significantly increases sedation and respiratory depression risk. MAO inhibitors must not be taken concurrently or within 14 days of using Phenergan.

• Children under 2 years of age (risk of fatal respiratory depression)
• History of hypersensitivity to promethazine or other phenothiazines
• Concurrent use with MAO inhibitors or within 14 days of stopping them
• Comatose states or severe CNS depression
• Narrow-angle glaucoma
• Urinary retention or symptomatic prostatic hypertrophy
• Epilepsy (use with great caution)
• Severe hepatic impairment