Some interactions make a medicine work less well. Other interactions make a medicine’s effects much stronger. Or they can cause side effects. For example, if you drink alcohol when you are taking acetaminophen, it can harm your liver. Sildenafil, prescribed for erectile dysfunction, and nitrites or nitrates can cause low blood pressure (hypotension). Food makers add nitrates and nitrites to processed sandwich meats, bacon, salami and sausages to add color and to lengthen shelf life. Low blood pressure can sometimes be fatal. The organ transplant medicine cyclosporine taken with St. John's wort can cause your body to suddenly reject the transplanted organ. Warfarin, a blood thinner, and either vitamin E or aspirin may cause too much bleeding.

These interactions can cause serious problems. It's important to read the label on any OTC medicine you take. The label includes information on possible medicine interactions. Medicine labels can change, so you should look at the label each time you buy a new supply. For example, taking an OTC antihistamine and a prescription 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 not sure about possible interactions, always check with your healthcare provider or pharmacist before taking the medicine.

Here are examples of a medicine-food interaction:

• Orange juice or a similar beverage can make your body absorb less nicotine from nicotine gum.
• Grapefruit juice should not be taken with some blood pressure and cholesterol medicines. It should also not be taken with cyclosporine, a medicine used by organ transplant patients.
• Dairy products, antacids, and vitamins with iron can make antibiotics not work as well as they should.
• 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. Having 3 or more drinks a day when taking acetaminophen can cause 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 get worse if you take a nasal decongestant or use a nasal spray. Antihistamines can make worse a problem with urination caused by prostate problems. Sleeping pills can make sleep apnea or emphysema worse.

When your healthcare provider prescribes a new medicine, tell your provider about all other medicines you take. This includes OTC and prescription medicines, dietary supplements, vitamins, botanicals, minerals, and herbals. Also ask your provider if the new medicine could interact with foods you normally eat. Most providers give you information on new prescription medicines. You can also read the medicine information insert that comes with prescriptions filled at your pharmacy. Pharmacists can also explain possible OTC and prescription medicine interactions.

Using the same pharmacy lets the pharmacist check for interactions.

OTC medicine labels can change, so be sure to read the label each time you buy an OTC product. Medicine labels contain information about ingredients, uses, warnings, side effects, and directions. They also tell you about possible medicine interactions. Different OTC medicines may contain the same active ingredient. If you are taking more than 1 OTC medicine, pay attention to the active ingredients used in the products. This will help prevent taking too much of a particular ingredient. For example, the common pain reliever acetaminophen is found in more than 600 prescription and OTC medicines. Acetaminophen can cause serious liver damage if you use more than directed. Antacids with calcium can interfere with how thyroid hormone is absorbed.

Read the label first. Call your pharmacist or healthcare provider if you have questions about whether it's safe to take with other medicines, vitamins, or herbal products you already take. Check that your provider and pharmacist know all of the current OTC and prescription medicines you use so they can give you accurate advice.

In the case of a serious reaction, call 911. For mild to moderate interactions, check with your healthcare provider right away. Don't stop taking prescription medicines until you check with your provider.