Like oil and water (or say, Mets and Yankee fans), there are some foods and drugs that just don't mix. What you do or don't take your pills with can determine not only the effectiveness but also the safety of the medications you're taking. "There are only a few food and drug interactions that patients need to worry about," says Daniel Albrant, Pharm.D., president of Pharmacy Dynamics, a health-care consulting business focused on improving medication use based in Arlington, Virginia. "The biggest problems tend to be that people don't take their drugs consistently and miss doses or double-dose to make up for it."

Still, there are a few rules to follow when taking medication. "You should always take medications, whether over-the-counter or prescription, with an eight-ounce glass of water—not coffee, alcohol, juice, or any other beverage," says Jan K. Hastings, Pharm.D., assistant professor at the University of Arkansas for Medical Sciences College of Pharmacy. There are several reasons for sticking to water: If you take enteric-coated pills (coated so that they dissolve in the intestines, not the stomach) with beverages other than water, the coating may dissolve before the pill reaches the intestines, which could result in stomach irritation or bleeding. Other liquids can make drugs more or less effective or heighten side effects. In particular, you shouldn't take medications with grapefruit juice: Because grapefruit juice is metabolized by the same enzyme that metabolizes certain medications—for example antifungal drugs ending in conazole—it's possible for the drug to become more potent than it should be.

However, pharmacists disagree about how dangerous the grapefruit juice effect is; Albrant says, "There's been a lot of hype about grapefruit juice interactions, but for the most part, the studies were done using larger amounts of juice than most people ever consume." However: "It's better to be safe than sorry," says Hastings. "Though it might be enough to just avoid taking medication within an hour of drinking grapefruit juice, I recommend that people avoid drinking grapefruit juice until the course of treatment is over."

You're also not doing yourself any favors by swallowing pills dry or with just a little sip of water, so drink a full eight ounces. "To get some drugs through your esophagus and into your stomach, you'll need a full glass of water," says Hastings. "If a pill just sits in your esophagus, it could cause problems—some could cause esophageal erosion or ulcers."

As for the instruction "take with food," it's best to check with your pharmacist to find out exactly what kind and how much. Some medications can be taken with a snack—say, a few crackers—and others should be taken with a full meal. "The reason you want to take most medications with food is to prevent stomach upset," says Hastings. "When your stomach is completely empty, it's at its most acidic. When you eat some food, the acid level goes down and there's a buffering effect." Some drugs need food present in order to be properly absorbed by the body, while others are best taken on an empty stomach for full potency.

Here are a few common medications and the foods that should be avoided when taking them.


Allergy medications

Over-the-counter allergy medications, like Benadryl, can make you sleepy, so avoid drinking alcohol—a depressant—while you're on them, or you won't be able to stay awake. "Food interactions with prescription allergy medications like Claritin or Allegra are rare," says Albrant. But watch out if you're taking the combination allergy and decongestant forms—the decongestant is a stimulant, so you'll want to avoid caffeine, which may make you bounce off the walls. "Some people react to decongestant products strongly, even without additional caffeine," warns Albrant. "You add caffeine, another stimulant, and you may feel like you're going to explode. It's not all in your head; your blood pressure and heart rate really do increase."


Antibiotics

Most antibiotics are best taken on an empty stomach, either at least an hour before meals or two hours after. Tetracycline and ciprofloxacin should not be taken within an hour of dairy products; liquid antacids; or calcium, iron, or magnesium supplements because these products bind with the antibiotics, making them less effective. Be particularly vigilant about avoiding alcohol when you're on a course of metronidazole—the combination of the two can cause stomach pain, vomiting, and headaches.


Anticoagulants

If you're taking a blood thinner, you've probably been told to avoid foods containing large amounts of vitamin K, like asparagus, broccoli, Brussels sprouts, cabbage, cauliflower, collard greens, kale, and spinach, because vitamin K makes your blood more likely to clot. "In fact, it's the change in quantities that's important," says Hastings. "If you were to eat a large amount of green vegetables all the time, that would be fine—the level of the blood thinner would be normalized for the amount of greens you're eating. Be sure to keep your doctor informed of diet changes." And avoid herbal supplements altogether—according to Hastings, garlic, ginseng, gingko, and guarana are all known to make you bleed easier, but there may be many more herbs that effect your bleeding time as well.


Antidepressants

The main thing to avoid with antidepressants is alcohol. "It's not that you're likely to have a dangerous reaction, but why take a depressant, which is what alcohol is, when you're depressed?" says Albrant.


Cough Suppressants

These products can make you drowsy, so you'll want to avoid alcohol, which could make you even sleepier, explains Albrant.


Decongestants

As with decongestant-containing allergy products above, lay off the double espressos or you'll risk a major case of the jitters.


Over-the-counter analgesics

The most important interaction to avoid with over-the-counter painkillers like acetaminophen, ibuprofen, or aspirin, is chronic use of alcohol (defined as three or more alcoholic drinks a day). "The liver has to metabolize both the alcohol and the analgesic, and over time there can be a buildup of toxic substances—which can cause liver damage," explains Albrant. "If you're regularly drinking three glasses of alcohol a day, you shouldn't be using these products. Just because they are over-the-counter doesn't mean they are without risk." So what should you do if you've had a few drinks and you've got a hangover? "A hangover really comes from dehydration, so the best thing you can do is have a glass of water before you go to sleep," says Albrant.