Some interactions make a medicine not work as well. Other interactions make a medicine’s effects much stronger. Or they can cause side effects. For instance, 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). 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.

Problems often happen between prescribed medicines and OTC medicines. 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 instance, 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 not sure about possible interactions, always check with your health care provider or pharmacist before taking the medicine.

Here are examples of a medicine-food interaction:

• Orange juice or a similar drink 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 high blood pressure reaction.

Drinking alcohol when taking a sedative, for instance, can make you feel tired or slow your reaction time. Having 3 or more drinks of alcohol 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. If you have a problem with peeing caused by prostate issues, taking antihistamines can make this worse. Sleeping pills can make sleep apnea or emphysema worse.

When your healthcare provider prescribes a new medicine, tell them about all other medicines you take. This includes OTC and prescription medicines, dietary supplements, vitamins, botanicals, minerals, and herbal products. Also ask your provider if the new medicine could interact with foods you normally eat.

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 one OTC medicine, pay attention to the active ingredients used in the products. This will help prevent taking too much of a particular ingredient. For instance, 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. If you're taking prescription medicines, the safest action is to ask your pharmacist about possible medicine interactions before using any new OTC product.

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.