Some interactions reduce a drug's effectiveness; others can dramatically increase a drug's effect or cause side effects. For example, alcohol consumed along with acetaminophen can damage the liver. Viagra and nitrites or nitrates can cause low blood pressure (hypotension), a sometimes fatal condition. The organ transplant drug cyclosporin taken with St. John's wort can cause sudden rejection of the transplanted organ. Warfarin, a blood thinner, and either vitamin E or aspirin may cause hemorrhaging.

Problems often occur between prescribed medication and OTC drugs. It is important to read the label on any OTC drug you take. The label includes information on possible drug interactions. Drug labels can change, so it's critical to look at the label each time you buy a new supply. For example, taking an antihistamine and a sedative at the same time can make you very drowsy. This can be dangerous if you are driving or need to operate machinery. If you are uncertain about possible interactions, always check with your health care provider or pharmacist before taking the drug.

Here are examples of a drug-food interaction: Orange juice or other acidic beverage can hinder the absorption of nicotine from nicotine gum. Grapefruit juice should not be taken with some blood pressure medications or cyclosporin (taken by organ transplant patients). Dairy products, antacids, and vitamins with iron can make antibiotics less effective. Eating cheese or drinking wine while taking a monoamine oxidase inhibitor may cause a severe blood pressure reaction.

Drinking alcohol when taking a sedative, for example, can make you feel tired or slow your reaction time, the FDA says. Having three or more drinks a day when taking acetaminophen can lead to liver damage. Drinking alcohol when taking aspirin, ibuprofen, or naproxen may cause ulcers.

If you have kidney disease, your condition could be made worse if you take ibuprofen. If you have high blood pressure, your condition could worsen if you take a nasal decongestant or use a nasal spray. Antihistamines can worsen a problem with urination caused by prostate problems. Sleeping pills can complicate sleep apnea or emphysema.

When your doctor prescribes a new drug, the FDA says, discuss all OTC and prescription drugs, dietary supplements, vitamins, botanicals, minerals, and herbals you take, as well as the foods you eat. Any of these may interact with the new medication.

Doing so lets your pharmacist check for interactions.

OTC drug labels can change, so be sure to read the label each time you buy an OTC product. Drug labels contain information about ingredients, uses, warnings, and directions, as well as information about possible drug interactions, the FDA says. Different OTC drugs may contain the same active ingredient. If you are taking more than one OTC drug, pay attention to the active ingredients used in the products to avoid taking too much of a particular ingredient.

Read the label first, and call your pharmacist or health care provider if you have questions about whether it's safe to take with other medications, vitamins, or herbal products you already take.

In the case of a serious reaction, call 911.