Taking prescription drugs and over-the-counter or alternative medications without knowing how they interact

It's easy to think that something you can grab off the shelf at your local grocery or drug store must be safe, but some of the most common OTC drugs can cause serious reactions. A top contender is medicine-chest staple Maalox, meant to calm digestive upset. A new and very popular version, Maalox Total Relief, contains an ingredient called bismuth subsalicylate that can react dangerously with anticlotting drugs, drugs for hypoglycemia, and anti-inflammatories, particularly ibuprofen and other nonsteroidal anti-inflammatories, or NSAIDs.

Another standby to watch out for is aspirin, which thins the blood. If you forget to stop taking aspirin before a surgical procedure, the result can be life-threatening bleeding.

Then there's the herb Saint-John's-wort, which many people take for depression. The fact that Saint-John's-wort can interfere with prescription antidepressants has received a fair amount of attention, but few people know that it also interferes with the liver's processing of blood thinners such as Coumadin (warfarin) and heart medications such as Digoxin.

How to avoid it: When your doctor is writing out a new prescription, this is also the time to mention or remind her about any OTC meds or supplements you take. Never add a medication without discussing how it interacts with what you're already taking.