Promethazine DM is a prescription cough syrup that combines two active ingredients: promethazine, an antihistamine, and dextromethorphan, a cough suppressant. It’s used to temporarily relieve coughs and upper respiratory symptoms caused by the common cold, allergies, or the flu, including runny or stuffy nose and itchy, watery eyes. The “DM” in the name stands for dextromethorphan.
What’s in It and How It Works
Each 5 mL dose (one teaspoon) contains 6.25 mg of promethazine hydrochloride and 15 mg of dextromethorphan hydrobromide. These two ingredients tackle cold and allergy symptoms from different angles.
Promethazine is a first-generation antihistamine. It blocks histamine receptors in your body, which reduces the runny nose, sneezing, and itchy eyes that come with allergies and upper respiratory infections. It also has mild sedating properties, which is why drowsiness is one of its most noticeable effects. Dextromethorphan works in your brain to suppress the cough reflex, quieting the persistent, nonproductive cough that often accompanies colds.
Together, the combination addresses both the cough itself and the surrounding congestion and allergy-type symptoms that make you miserable when you’re sick.
How It’s Taken
Promethazine DM comes as an oral liquid solution. The standard adult dose is one teaspoon (5 mL) every 4 to 6 hours, with a maximum of 30 mL (six teaspoons) in a 24-hour period. It requires a prescription; you can’t buy it over the counter. However, it is not classified as a controlled substance.
Common Side Effects
The most frequently reported side effects come from the promethazine component. Drowsiness is the big one, and it can be significant enough to impair your ability to drive or operate machinery. Confusion and disorientation are also possible, especially at higher doses or in older adults.
Because promethazine has anticholinergic effects (meaning it blocks certain nerve signals throughout your body), it can also cause blurred vision, dry mouth, dry nasal passages, constipation, and difficulty urinating. These effects are generally mild but can be uncomfortable. In some people, promethazine paradoxically causes the opposite of sedation: restlessness, excitability, or agitation.
Serious Risks to Know About
Promethazine DM carries an FDA boxed warning, the most serious type of safety alert. The medication must not be used in children under 2 years old because of the risk of fatal respiratory depression, meaning it can slow breathing dangerously or stop it entirely. Postmarketing reports have documented deaths in this age group across a wide range of doses. Even in children 2 and older, caution is warranted because the risk of respiratory depression remains.
Rare but serious side effects in adults include abnormal heart rhythms, low blood pressure, and involuntary muscle movements (such as muscle stiffness, tremors, or repetitive motions). These movement-related effects stem from promethazine’s impact on dopamine signaling in the brain. In very rare cases, promethazine has been associated with a dangerous reaction that involves high fever, severe muscle rigidity, confusion, and unstable blood pressure and heart rate. Liver damage and drops in white blood cell counts have also been reported, though these are uncommon.
Interactions With Other Substances
Because both ingredients in promethazine DM can cause sedation and slow brain activity, combining this medication with alcohol, opioid painkillers, sleep aids, or anti-anxiety medications amplifies those effects. The combination can dangerously increase drowsiness, impair coordination, and further suppress breathing. If you’re taking any medication that causes drowsiness, that interaction is worth flagging before starting promethazine DM.
Dextromethorphan can also interact with certain antidepressants, particularly SSRIs and MAO inhibitors. Taking dextromethorphan alongside an MAO inhibitor can trigger a dangerous spike in serotonin levels, so these drugs should not be used together.
Who Should Avoid It
Beyond the strict contraindication in children under 2, promethazine DM is not appropriate for people with severe breathing problems like asthma or chronic obstructive lung disease, since both ingredients can suppress respiratory function. People with liver disease, seizure disorders, or a history of reactions to antihistamines in the phenothiazine family should also avoid it. The sedating effects can be more pronounced and unpredictable in older adults, making lower doses or alternative medications a better fit for that group.