If you’re like most people, you down your pills with juice or food, thinking you’re doing your stomach a favor by not taking the medications alone. As it turns out, you may be doing more harm than good due to food-drug interactions.

Recent research shows that foods and beverages can have a dramatic effect on how the body absorbs certain medications.

“Twenty years ago, a team of researchers and I realized that grapefruit juice could cause many medications to be as much as 10 times more powerful than they’re meant to be,” says David G. Bailey, PhD, clinical professor of pharma­col­ogy at the University of Western Ontario in Canada.  

Thanks to their research, more than 50 medications now carry cautions about grapefruit juice on their labels.

Now Bailey has discovered that grape­fruit, apple and orange juices can actually have the opposite effect on some other drugs. “They block the pills’ absorption, so you get less or even no benefit from taking them,” says Bailey. “This might cause someone to think that they need to switch to a ‘better’ medication – when in fact changing what they eat, or when, would make the drug effective.”

Here are five common pill blockers to watch out for:

FOOD: Milk and yogurt
BLOCKS: Iron supplements; many antibiotics, including fluoroqui­no­lone, cipro flo­xacin and “cycline” anti­biotics like tetracycline; thyroid hormone; and penicillamine (Cupri­mine, Depen), a disease-modifying anti­rheumatic drug

FOOD: Apple, orange and grapefruit juice
BLOCKS: Disease modifying anti-rheumatic drugs, including metho­trexate (Theu­ma­trex, Trexall); cyclo­spor­ine (Gengraf, Neoral, San­dim­mune), a drug used for severe psoriasis and rheumatoid arth­ritis; beta blockers like acebutol (Sectral); cancer drugs like etoposide (Eto­pophos, Vepesid); alendronate (Fosamax), an osteoporosis drug; the allergy medicine fexofenadine (Allegra); some antibiotics including ciprofloxine (Cipro) and levoflox­acin (Levaquin)

FOOD: Coffee
BLOCKS: Alendronate; the antibiotic penicillin

FOOD: Foods rich in vitamin K, including leafy green vegetables and liver
BLOCKS: Blood thinners like warfarin (Coumadin)

FOOD: Alcohol
BLOCKS: Anti­depres­sants, especially mono­amine oxi­dase inhib­itors (MAOIs) like tran­yl­cypromine (Par­nate) – although people taking SSRIs like fluoxetine (Prozac) should avoid drinking more than moderate amounts of alcohol as well, as it counteracts their benefits.

How long should you wait?

In most cases, you can consume a potentially pill-blocking food or beverage about three to four hours after you’ve taken your meds, but it’s best to ask your pharmacist, who can tell you for sure. Plus, she can check to see if other medications or supplements you take might further affect how your body absorbs your medicines.