By Chris Woolston

updated 8:26 a.m. ET Dec. 18, 2007

Viewers of the medical drama "House" have come to expect bizarre explanations for every symptom: A sudden case of the sweats might be the work of a 20-foot tapeworm. Or a hidden brain tumor. Or a 20-foot tapeworm with a hidden brain tumor.

If "House" mirrored real life, many episodes would center around a mundane culprit: a drug interaction. As Americans take more and more medications for everything from skittish stomachs to sluggish moods, clashes between drugs have fueled a new epidemic of unexpected, sometimes dangerous side effects and complications.

One recent study suggested that at least 1.3 million Americans have prescriptions for drugs that could cause problems if taken together — and that only counts people with health insurance. Although the overall toll is unknown, it's undoubtedly huge, experts say ... and growing.

Of course, if you take many medications at the same time, or large doses of a few, you're more likely to run into a bad pairing, says Marietta Anthony, PhD, associate director of the Center for Education Research and Therapeutics at the University of Arizona. But even common items like drugstore pain relievers can clash with other meds. Scan our list: If you spot your prescription, be extra alert for signs of possible conflicts; then, talk to your doctor. And check our easy switches — you may be able to take a safer combo.

If you take...

An SSRI for depression

Selective serotonin reuptake inhibitors such as fluoxetine (Prozac), paroxetine (Paxil), or sertraline (Zoloft)

One possible conflict: A triptan drug used to treat migraines, such as sumatriptan (Imitrex), naratriptan (Amerge), or zolmitriptan (Zomig)

Why: Triptans and this type of antidepressant both increase levels of the brain chemical serotonin. But too much serotonin can set off a chemical firestorm known as serotonin syndrome, causing mania, an increased heart rate, seizures, and even death. The syndrome is rare, but the threat is real, says John Horn, PharmD, a professor of pharmacy at the University of Washington and coauthor of The Top 100 Drug Interactions. In 2006, the FDA issued an advisory to the roughly 50,000 Americans who take both SSRIs and triptans. The advisory told users they didn't have to stop taking the drugs but cautioned them to be aware of the risk.

Protect yourself: Most patients can continue taking both triptans and SSRIs as long as they watch for signs of serotonin syndrome, Horn says. If you notice troubling symptoms, stop taking the triptans and check with your doctor right away.

Another possible conflict: Chronic use of nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin or ibuprofen.

Why: You may have heard that long-term use of NSAIDs can cause bleeding ulcers and other stomach troubles. What's less widely known is that adding an SSRI increases the risk. Serotonin is the culprit here, too. Normally, that hormone encourages blood platelets to stick together —but because platelets soak up less serotonin when you're on an SSRI, they may have trouble performing their number one job: clumping together to form clots and prevent excessive bleeding in the stomach and elsewhere.

Protect yourself: Even if you take an SSRI, it's fine to pop a couple ibuprofen or aspirin for an occasional headache. But if you need several doses of a pain reliever each day — a common regimen for people with arthritis--take acetaminophen instead. It's not an NSAID, and it doesn't encourage bleeding.

If you take...

Blood pressure medication

Specifically ACE inhibitors, such as benazepril (Lotensin), or diuretics, such as furosemide (Lasix) or hydrochlorothiazide (HCTZ)

Possible conflict: Chronic use of NSAIDs, such as aspirin or ibuprofen

Why: If you take daily NSAIDs, your painkillers could keep these kinds of blood pressure medications from doing their job. These drugs work by ridding the body of extra salt or water, or by shutting off production of a hormone that prompts blood vessels to narrow. If taken regularly, NSAIDs can block both of these actions--and the blood pressure benefit.

Protect yourself: If you experience problems, you have two options (ask your doctor which is right for you). "You can switch to a different blood pressure medication," Horn says: BP drugs called calcium channel blockers (such as amlodipine, or Norvasc) aren't deterred by NSAIDs. Or swap the pain relievers--use acetaminophen instead.

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