Cold & Flu Medicine, Decoded:
What Works and What Not to Mix

The cold and flu aisle looks like it holds a hundred different medicines. It actually holds about a dozen ingredients, mixed and renamed in different combinations. Once you can see the ingredients, most of the danger — and most of the wasted money — becomes obvious.

This matters more than it seems. The biggest risks in cold season rarely come from one product. They come from two or three products taken together that quietly contain the same thing, or from an ingredient that clashes with a medicine you already take every day.

1. Read the ingredients, not the brand

Every over-the-counter medicine in the US carries a standardised Drug Facts label, and the FDA requires the active ingredients — with the amount in each dose — to be listed first. That box is the only reliable part of the packaging. Brand families reuse the same name across very different formulas, so "Severe", "Nighttime", "PE" or "D" on the front can mean a completely different set of ingredients inside.

Nearly every multi-symptom product is built from the same five jobs:

If "active ingredient" versus "inactive ingredient" is new territory, our primer on what a medication ingredient actually is covers the basics.

2. Phenylephrine: the decongestant that doesn't work

Three details matter in practice:

• Only the pill form is affected

  The FDA's proposal applies to phenylephrine taken by mouth. Nasal sprays are not part of it.

• The other ingredients still work

  If you bought a combination product, the FDA notes that phenylephrine's presence does not affect how the other active ingredients work. The pain reliever or cough suppressant in the same capsule still does its job.

• There are alternatives — with conditions

  The FDA lists oral pseudoephedrine, decongestant nasal sprays, nasal corticosteroids, and nasal antihistamines as other OTC options for congestion. Pseudoephedrine is sold behind the pharmacy counter with photo ID and federal purchase limits of 3.6 grams a day and 9 grams in 30 days. Decongestant sprays work quickly, but follow the day limit on the label — using them longer can cause rebound congestion.

A practical shortcut: products labelled "PE" generally contain phenylephrine. The original formulations kept behind the counter contain pseudoephedrine. The active ingredient on the Drug Facts label is what tells you which one you have.

3. The acetaminophen trap

This is the most important safety point in the whole aisle. According to the FDA, more than 600 prescription and over-the-counter medicines contain acetaminophen, and taking too much of it can cause severe liver damage.

Prescription pain relievers are a common blind spot here, because some combine acetaminophen with another drug. Our guide to the ingredients inside common painkillers shows which ones do, and our piece on mixing alcohol with medication covers why a few drinks change the picture.

4. Decongestants and high blood pressure

Oral decongestants — pseudoephedrine and phenylephrine — work by narrowing blood vessels. That dries your sinuses, and it also raises blood pressure. The American Heart Association notes that its 2017 blood pressure guidelines with the American College of Cardiology list both decongestants and NSAIDs, found in many cold medicines, as medications that can increase blood pressure.

• Who should be most careful

  The biggest concern is for people who have had a heart attack or stroke, or who have heart failure or uncontrolled high blood pressure. Experts quoted by the AHA advise that people with uncontrolled high blood pressure or heart disease avoid oral decongestants.

• If you do use one

  Guidelines suggest the shortest possible course, and the AHA article advises not taking decongestants for more than seven days without speaking to a healthcare provider. Nasal saline or an antihistamine are suggested alternatives for congestion.

• Watch the pain reliever too

  Oral NSAIDs such as ibuprofen and naproxen can also raise blood pressure. For pain and fever, the guidance points to acetaminophen or topical NSAIDs as alternatives — which brings you back to the acetaminophen rules above.

Some products are marketed as suitable for people with high blood pressure and leave the decongestant out. Check the Drug Facts label rather than trusting the box name. And if you take blood pressure medication, a cold is a good moment to keep it on schedule — we looked at the evidence on taking blood pressure pills in the morning or at night.

5. Night-time formulas and sedating antihistamines

Most "night" and "PM" cold products get their drowsiness from a first-generation antihistamine, usually diphenhydramine or doxylamine. Their labels warn about marked drowsiness, about alcohol and sedatives making it worse, and about care when driving or operating machinery.

6. Children are not small adults

The FDA is direct on this: children under two should not be given any cough and cold product that contains a decongestant or antihistamine, because serious and possibly life-threatening side effects could occur. Manufacturers also voluntarily label these products "do not use in children under 4 years of age."

For older children, use only products made for children, dose by the label, and use the measuring device that came in the box rather than a kitchen spoon. When in doubt, a pharmacist can tell you in a minute whether a product is appropriate for your child's age and weight.