Surprising Side Effects of 10 Over-the-Counter Medicines

by James Hubbard, MD, MPH

Every medication has potential side effects. Every one. That includes all herbs, supplements, and “natural” medicines. Anything you ingest, rub on, or inhale that treats your problem can have negative effects as well.

The trick is finding the ones with the best positive effects and least potential negatives.

The medicines I cite below are some that fall into that category. That’s one of the reasons the regulators have approved them for over-the-counter use. But like one of my professors once said (to paraphrase), it doesn’t really matter if you have a one-in-a-million chance of getting something if you’re the one who gets it.

Here are some of the medication side effects you might encounter in common over-the-counter products. They’re not the only effects these meds can cause; they’re just some of the more common. Some will probably surprise you. And, of course, if you encounter an adverse effect, many times stopping the medicine (with the advice of your doctor if appropriate) will make the side effect go away.

As you look through this list, you might want to pay special attention to medications you’re more likely to take. Did you know about these possible side effects? Have you ever experienced them?

(Important: Other things can also cause these symptoms. This list is for general information, not diagnosis.)

Why Should I Care?

If you’re aware of common side effects, you can better choose which over-the-counter remedy is best for you. You may decide not to take one, to talk to your doctor, or to be ready to handle the risks.

For example, if you have trouble with bleeding or stomach pain, aspirin may not be the best pain reliever for you. Or if you’ll be driving or you have an enlarged prostate, you’ll want to consider the consequences before taking antihistamines.

Also, if you have some onset of new symptoms, you can consider whether what you’re taking could be causing them.

1. Antihistamines

These include diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), loratadine (Claritin), and cetirizine (Zyrtec). Antihistamines may cause:

  • Headaches
  • Increased asthma problems
  • Drowsiness (even the “nonsedating” ones can do this, but they’re less likely to)
  • Urine retention in men with enlarged prostates (less likely with the nonsedating ones)
2. Decongestants

Medicines in this category include pseudoephedrine (Sudafed), phenylephrine (Sudafed PE), and the multiple cold medicines that contain a combination of decongestants and antihistamines. Decongestants can cause:

  • Nervousness or insomnia
  • Aggravation of glaucoma, heart, diabetes, and thyroid problems
  • Urinary retention in men with an enlarged prostate
3. Decongestant nose spray

The nose spray version of decongestants can cause all the side effects of the oral decongestants, but it’s less likely to do so. However, using the spray for several days in a row can cause a rebound withdrawal so that when you stop using it, your nasal passages swell back up even if you’re over the illness.

4. Pepto-Bismol

May cause a black tongue or black bowel movements. There’s no health risk to this, when caused by Pepto-Bismol, but it could cause unnecessary concern.

5. Proton-pump inhibitors

These stomach-acid drugs, such as Nexium and Prilosec, can cause:

  • Headaches
  • Diarrhea
  • A decreased ability to absorb calcium (increasing risk for osteoporosis)
6. H2 blockers

H2 blockers, which are also used for stomach-acid problems, include cimetidine (Tagamet), famotidine (Pepcid), nizatidine (Axid), and ranitidine (Zantac). They can cause:

  • Headaches
  • Dizziness
  • Diarrhea

Cimetidine can cause breast tenderness or swelling in men.

7. Aspirin
  • Increases risk of stomach irritation (gastritis), ulcers, and any kind of bleeding. (Risk increases further if aspirin is taken with alcohol.)
  • May increase uric acid in people with gout.
8. Nonsteroidal anti-inflammatory drugs (NSAIDs)

The over-the-counter ones other than aspirin include ibuprofen (Advil) and naproxen (Aleve). NSAIDs can:

  • Irritate the stomach and cause ulcers and bleeding
  • Decrease the effectiveness of aspirin if you’re taking it to prevent heart problems
  • Increase blood pressure and fluid retention
  • Damage the liver
9. Aluminum/magnesium antacids

Maalox is a popular one. These drugs may:

  • Cause diarrhea, constipation, headache
  • Decrease your body’s ability to absorb calcium and certain prescription medicines
10. Calcium antacids

Tums falls into this category. Calcium antacids may cause:

  • Constipation
  • An increased risk of gallstones
  • Some rebound stomach acid after stopping the medicine


And there they are. No doubt you’ve taken one of these lately or have it in your first-aid kit.

Now, don’t be like many of my patients (or me) and read about side effects and all of a sudden imagine you have them all. But if one of these problems does suddenly develop, perhaps you should think about whether an over-the-counter medicine you’re taking could be causing it.

What about you? Have you ever had any of these medication side effects, or others I didn’t mention?

You may also be interested in “The Dangers of Acetaminophen (Tylenol)—Put Into Context.”