Promethazine is a medication used to treat allergies, motion sickness, nausea, vomiting, pain after surgery and anxiety before surgery. It is also used as a sleep aid. It is a first-generation antihistamine that belongs to the drug class called phenothiazines; it prevents the release of histamine from specific cells in the body. 

This substance is released in response to things you are allergic to, such as mould, pollen, chemicals, dander, etc. The drug induces sedation and controls pain because histamine helps you feel awake and alert with heightened senses. 

Dosage

Promethazine comes in different forms: oral tablets, injectibles, oral solutions, and rectal suppositories. This dosage information is for oral tablets only, available in strengths of 12.5 mg, 25 mg, and 50 mg. 

The dosage for allergies is 25 mg taken at bedtime or 12.5 mg taken before meals and at bedtime. As symptoms start improving, your doctor may reduce the dosage until it reaches the smallest amount that works.

It has not been confirmed if promethazine is safe for children below 2 years old, but older adults who are 65 years and above can start with lower doses as prescribed by a doctor.  

The dosage for motion sickness is 12.5 mg twice daily, taken 30 minutes to 1 hr before travelling and the second dose 8 hrs later. The dosage for nausea and vomiting is 25 mg, and it can be retaken after 4 to 6 hours if it is still needed. 

Side Effects of Promethazine 

The common side effects of promethazine include:

• Allergic skin reactions.
• Drowsiness.
• Abnormal movements.
• Breathing problems.
• Reduced white blood cell production.
• Changes in blood pressure.
• Increased excitability.
• Reduced platelet counts.

These side effects are usually mild, but if they don’t go away after a few days, talk to your doctor about it. The medication can also cause some more severe side effects, including:

• Uncontrollable movements.
• Seizures.
• Abnormal heart rhythms.
• Hallucinations.
• Severe breathing problems.
• Neuroleptic malignant syndrome.
• Angioedema.
• Decreased platelet and white blood cell production.
• Jaundice. 

Precautions

Promethazine may cause severe allergic reactions with symptoms like difficulty breathing and swelling of the throat and tongue.

It can react with alcohol and make drowsiness last longer, so don’t take alcohol when taking the drug.

Also, do not drive a car or use machinery after taking the drug because it will make you drowsy. Let your doctor know if you plan on getting pregnant or are already pregnant and if you are breastfeeding. 

Also, let your doctor know if you have any of the following medical conditions:

• Breathing problems.
• Bone marrow disease.
• Stomach problems.
• Bladder problems.
• Angle-closure glaucoma.
• Enlarged prostate.
• Heart disease.
• Liver disease.
• Seizures.
• Sleep apnea.