chemistry

Promethazine

Promethazine is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Promethazine rather than just read about it. In short: Promethazine, sold under the brand name Phenergan among others, is a first-generation antihistamine, sedative, and antiemetic used to treat allergies, nausea, and vomiting. It may also help with some symptoms associated with the common cold and may also be used for sedating people who are agitated or anxious, an effect that has led to some recreational use (especially with codeine).

Key takeaways

Promethazine belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Promethazine to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Promethazine from memory before moving on to harder problems.

Reference excerpt

Promethazine, sold under the brand name Phenergan among others, is a first-generation antihistamine, sedative, and antiemetic used to treat allergies, nausea, and vomiting. It may also help with some symptoms associated with the common cold and may also be used for sedating people who are agitated or anxious, an effect that has led to some recreational use (especially with codeine). Promethazine is taken by mouth (oral), as a rectal suppository, or by injection into a muscle (IM). Common side effects of promethazine include confusion and sleepiness; consumption of alcohol or other sedatives can make these symptoms worse. It is unclear if use of promethazine during pregnancy or breastfeeding is safe. Use of promethazine is not recommended in those less than two years old, due to potentially negative effects on breathing. Use of promethazine by injection into a vein is not recommended, due to potential skin damage. Promethazine is in the phenothiazine family of medications. It is also a strong anticholinergic and at high or toxic doses can act as a deliriant. Promethazine was made in the 1940s by a team of scientists from Rhône-Poulenc laboratories. It was approved for medical use in the United States in 1951. It is a generic medication and is available under many brand names globally. In 2023, it was the 230th most commonly prescribed medication in the United States, with more than 1 million prescriptions; and the combination with dextromethorphan was the 252nd most commonly prescribed medication in the United States, with more than 1 million prescriptions.

Medical uses

Promethazine has a variety of medical uses, including:

Sedation In Germany, it is approved for the treatment of agitation and agitation associated with underlying psychiatric disorders with a maximum daily dose of 200 mg. For nausea and vomiting associated with anesthesia or chemotherapy. It is commonly used postoperatively as an antiemetic. The antiemetic activity increases with increased dosing; however, side effects also increase, which often limits maximal dosing. For moderate to severe morning sickness and hyperemesis gravidarum: In the UK, promethazine is the drug of first choice. Promethazine is preferred during pregnancy because it is an older drug and there is more data regarding the use of it during pregnancy. Second-choice medications, which are used if promethazine is not tolerated or the patient cannot take it, are metoclopramide or prochlorperazine. For allergies such as hay fever and together with other medications in anaphylaxis To aid with symptoms of the common cold Motion sickness, including space sickness Hemolytic disease of the newborn Anxiety before surgery May be used to treat cannabinoid hyperemesis syndrome

Side effects Some documented side effects include:

Tardive dyskinesia, pseudoparkinsonism, acute dystonia (effects due to dopamine D2 receptor antagonism) Confusion in the elderly Drowsiness, dizziness, fatigue, more rarely vertigo Known to have effects on serotonin and dopamine receptors. Dry mouth Nausea Respiratory depression in patients under the age of two and those with severely compromised pulmonary function Blurred vision, xerostomia, dry nasal passages, dilated pupils, constipation, and urinary retention. (due to its anticholinergic effects) Chest discomfort/pressure (In children less than 2 years old) Akathisia Less frequent:

Cardiovascular side effects to include arrhythmias and hypotension Neuroleptic malignant syndrome Liver damage and cholestatic jaundice Bone marrow suppression, potentially resulting in agranulocytosis, thrombocytopenia, and leukopenia Depression of the thermoregulatory mechanism resulting in hypothermia/hyperthermia Rare side effects include:

Seizures Because of the potential for more severe side effects, this drug is on the list to avoid in the elderly. In many countries (including the US and UK), promethazine is contraindicated in children less than two years of age, and strongly cautioned against in children between two and six, due to problems with respiratory depression and sleep apnea. Promethazine is listed as one of the drugs with the highest anticholinergic activity in a study of anticholinergic burden, including long-term cognitive impairment.

Tissue necrosis risks specific to the injectable form Injectable promethazine is a vesicant and is capable of damaging blood vessel intima, potentially resulting in tissue injury and gangrene. A large number of severe complications have been reported following inadvertent arterial injection, subcutaneous injection, or extravasation of promethazine, including the amputation of fingers and a forearm. One lawsuit reached the Supreme Court of the United States and resulted in $6.7 million in damages. Severe injuries resulting from inadvertent arterial injections of promethazine have been known since at least the 1970s. When injectable promethazine must be used, the Institute for Safe Medication Practices recommends limiting the dose to between 6.25 mg or 12.5 mg, using central access sites and large veins, administering the drug slowly, or resorting to safer options.

Overdose Promethazine in overdose can produce signs and symptoms including CNS depression, hypotension, respiratory depression, unconsciousness, and sudden death. Other reactions may include hyperreflexia, hypertonia, ataxia, athetosis, and extensor-plantar reflexes. Atypically and/or rarely, stimulation, convulsions, hyperexcitability, and nightmares may occur. Anticholinergic effects, like dry mouth, dilated pupils, flushing, gastrointestinal symptoms, and delirium, may occur as well. Treatment of overdose is supportive and based on symptoms.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Promethazine

Start with the simplest possible case. Write down what Promethazine claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Promethazine before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Promethazine ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Promethazine

In research: Promethazine appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Promethazine in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Promethazine is common in secondary-school and first-year university syllabi. It links to neighbouring topics Antiemetics, Antimigraine drugs, CYP2C9 inhibitors, so understanding it makes those chapters shorter.
In everyday life: Look for Promethazine outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Promethazine in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Promethazine means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Promethazine out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Promethazine in simple terms?

Why does Promethazine matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Promethazine?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Promethazine.