The research: One hundred commonly prescribed drugs in the UK were studied, and 74 of them had lactose, gelatin, or magnesium stearate—ingredients that come from cows, pigs, and fish. Gelatin is often used in capsules or coatings, and lactose and magnesium stearate are often mixed with the active ingredients of the drugs.  What it means: The researchers say doctors and patients often aren’t aware that prescription drugs are made with animal by-products. Reading labels does not always help since they found many are unclear or inconsistent.  Animal-derived ingredients aren’t necessarily harmful, but they go against the preferences of vegans, vegetarians, and those with strict religious or cultural diets. Rarely, people with some intolerances or allergies may suffer side effects. And here’s the thing: Medications don’t need to use animal by-products. “It would be easy to use vegetarian versions of these ingredients,” says Kinesh Patel, MBBS, a research fellow at St. Mark’s Hospital in the UK and one of the study’s authors. Vegetarian magnesium sulphate and non-gelatin capsules are available, as well as lactose without cow enzymes. Vegan ingredients, on the other hand, would be more challenging to use, he adds.  The bottom line: Kelly Hogan, MS, RD, a dietitian at the Mount Sinai Hospital in New York, says patients with strong beliefs about animal derivatives should communicate that to their doctors. To find out if your meds contain animal derivatives, look at the leaflet that comes with them or go to the product’s website. If you can’t find the information there, contact the manufacturer or the U.S. Food and Drug Administration. Some common meds that use animal by-products are omeprazole (Prilosec), which can be encased in gelatin, and simvastatin (Zocor) and levothyroxine (Synthroid), which commonly contain lactose. Other drugs with versions of animal by-products are warfarin (Coumadin), citalopram (Celexa), and codeine. Aspirin, ibuprofen, and hydrocortisone may also contain animal by-products. More from Prevention: Is Your Medication Making You Sick?