Promethazine is a phenothiazine derivative, a class of compounds originally developed for psychiatric use. The active ingredient in all promethazine products is promethazine hydrochloride, a synthetic compound that blocks histamine receptors, dopamine receptors, and certain nerve signaling pathways in the brain. Beyond this active ingredient, what’s actually in your promethazine depends on whether you have tablets, a liquid syrup, a combination cough formula, or an injectable solution.
The Active Ingredient
Promethazine hydrochloride is the working compound in every formulation. It does three things in the body: it blocks histamine (the chemical behind allergy symptoms like sneezing, itching, and hives), it blocks dopamine signaling in the brain’s nausea center, and it interferes with a nerve signaling system called the cholinergic pathway. This combination of effects is why promethazine treats such a wide range of problems, from seasonal allergies and motion sickness to nausea, vomiting, and pre-surgical anxiety.
The histamine-blocking action is what makes it useful for allergies and itching. The dopamine-blocking action handles nausea. And the overall sedating quality, a product of all three mechanisms working together, is why it’s used to calm patients before surgery or help with sleep. Few single drugs cover this many bases, which is part of why promethazine has stayed in use for decades.
What’s in the Tablets
Promethazine tablets are the simplest formulation. Common tablet strengths are 12.5 mg, 25 mg, and 50 mg of promethazine hydrochloride. The inactive ingredients are straightforward binders and fillers: lactose monohydrate (a milk sugar that holds the tablet together), magnesium stearate (a lubricant that prevents the tablet from sticking to manufacturing equipment), hydroxypropyl cellulose, and low-substituted hydroxypropyl cellulose. These are standard pharmaceutical fillers found in many generic medications. If you have a lactose intolerance, the lactose monohydrate content is worth noting, though the amount per tablet is small.
What’s in the Liquid Syrups
Promethazine syrups come in two categories: plain syrup and combination formulas. The ingredients differ significantly between them.
Plain Promethazine Syrup
Plain promethazine syrup contains 6.25 mg of promethazine hydrochloride per teaspoon (5 mL) in a flavored syrup base. This is the version typically prescribed for allergies, nausea, or sedation on its own.
Promethazine With Codeine
This combination adds codeine phosphate, an opioid cough suppressant, at 10 mg per teaspoon alongside 6.25 mg of promethazine hydrochloride. The syrup base contains 7 percent alcohol by volume. That’s a meaningful amount of alcohol, roughly comparable to a light beer, and something to be aware of if you’re avoiding alcohol for medical, religious, or personal reasons.
Promethazine DM
The “DM” stands for dextromethorphan, a non-opioid cough suppressant. Each teaspoon contains 15 mg of dextromethorphan hydrobromide and 6.25 mg of promethazine hydrochloride. This version also contains 7 percent alcohol. The solution has a yellow color with a pineapple-menthol flavor. Because dextromethorphan is not an opioid, this formula doesn’t carry the same addiction risk as the codeine version.
Promethazine With Codeine and Phenylephrine
A triple-combination syrup adds phenylephrine hydrochloride (5 mg per teaspoon), a nasal decongestant, to the codeine (10 mg) and promethazine (6.25 mg). This version also sits at 7 percent alcohol. It targets cough, congestion, and runny nose simultaneously.
What’s in the Injectable Form
The injectable version, used in hospitals and clinical settings, contains additional stabilizers and preservatives not found in oral formulations. Each milliliter includes either 25 mg or 50 mg of promethazine hydrochloride along with sodium metabisulfite (0.25 mg, a preservative), edetate disodium (0.1 mg, a stabilizer), calcium chloride (0.04 mg), and phenol (5 mg, which acts as a preservative). The solution is buffered with acetic acid and sodium acetate to maintain a pH between 4.0 and 5.5. The sodium metabisulfite is notable because people with sulfite sensitivities can react to it.
What Promethazine Is Used For
The FDA has approved promethazine for a surprisingly broad list of conditions. For allergies, it treats seasonal and year-round nasal allergies, allergic eye inflammation, hives, and mild allergic skin reactions. It’s also approved as a supporting treatment during severe allergic reactions after the immediate crisis is controlled.
For nausea and vomiting, it works both as a treatment and as prevention, particularly around surgery and anesthesia. The typical dose for nausea is 25 mg, repeated every four to six hours if needed. For motion sickness, the standard approach is 25 mg taken 30 to 60 minutes before travel, with another dose 8 to 12 hours later.
As a sedative, promethazine is used before and after surgery, during labor, and for general calming in both adults and children. Adult sedation doses range from 25 to 50 mg, while children’s doses are calculated by weight at roughly 0.5 mg per pound of body weight.
The Pediatric Safety Warning
Promethazine carries a boxed warning, the FDA’s most serious safety label, for use in children under two years old. Fatal cases of respiratory depression (dangerously slowed breathing) have been reported in this age group. The drug is flatly contraindicated for children under two. For older children, doses are weight-based and kept as low as possible. The sedating effects that make promethazine useful in adults can overwhelm a young child’s ability to maintain normal breathing, which is what makes this age cutoff so firm.
Why the Alcohol Content Matters
All of the liquid combination syrups contain 7 percent alcohol. This is partly a solvent for the active ingredients and partly a preservative. If you’re taking other sedating medications, the alcohol adds to the drowsiness effect. It also means these syrups can interact with medications that don’t mix well with alcohol, including certain antidepressants, anxiety medications, and sleep aids. The plain tablet form avoids this issue entirely.