James Cagney had a thing for grapefruit. He stuffed a half into Mae Clarke’s face in the 1931 movie “Public Enemy”; in “Hard to Handle” his character, con man Lefty Merrill, went on an 18-day grapefruit diet. If he tried that diet today — while taking statins, some allergy pills or antibiotics — “Why, why, he’d lose more than a little weight, we tell ya.”

But seriously, 85 medications are known to interact with grapefruit and grapefruit juice; 42 have serious, possibly life-threatening interactions. Seems that grapefruit interferes with an enzyme that metabolizes many drugs, and that interaction increases (or decreases) the amount of medication in your bloodstream. If you take certain statins with grapefruit, for example, you end up with far too much of those meds in your system, increasing the risk of liver damage, muscle breakdown and kidney failure. Grapefruit can be hazardous if you eat or drink it four or even 12 hours after taking your meds.

So here’s how to make sure your medications do what they’re supposed to:

Æ Always read information provided by the pharmacy with a new prescription.

Æ For meds you’re taking, go see your pharmacist, medications in hand, and ask whether any of them interact with grapefruit or grapefruit juice.

Æ Avoid grapefruit if it interacts with your meds, or if you eat or drink the same amount every day, tell your doctor and he can adjust your meds’ dosage. Some docs do this to reduce both the dose and the cost of drugs.

Just tell ’em Lefty sent ya.

The You Docs — Mehmet Oz, host of “The Dr. Oz Show” and Mike Roizen of Cleveland Clinic — are authors of “YOU: Losing Weight.” To submit questions, go to www.RealAge.com. A King Features syndicate.