Taking supplements can support your overall health, but timing and combinations matter. Certain pairings can reduce absorption or trigger side effects.

What happens: Taking calcium with other minerals like iron isn’t a good idea. “Both calcium and zinc can block the absorption of iron supplements because they compete for absorption in the intestines,” Jennifer Warren, MD, DABOM, DABFM, medical director at Mochi Health, told Health. “This is especially important for people who have an iron deficiency who need extra iron to improve anemia,” she said.

Why it matters: Since iron and calcium compete for absorption in the digestive tracts, taking them together can lower their effectiveness by reducing the amount of these nutrients your body absorbs.

What to do instead: “To ensure the best absorption, take iron by itself in the morning with either water or citrus juice, since vitamin C can assist iron absorption, and avoid other mineral supplements for at least two hours,” recommended Warren. If iron supplements make you nauseated when taken on an empty stomach, you can take them with a small amount of food. Calcium should also be taken on its own to optimize absorption.

What happens: Like iron, calcium competes for absorption with other minerals, like zinc. “Calcium and zinc will compete for absorption in the gut, so taking them together can reduce how much you absorb of each,” C. Vivek Lal, M.D., director of clinical innovation at Heersink Institute of Biomedical Innovation at the University of Alabama at Birmingham, told Health.

Why it matters: “Zinc supports immune function and healing, while calcium helps fill dietary gaps to meet daily needs for bone health,” explained Lal. “Poor uptake from nutrient competition impacts these benefits,” he said.

What to do instead: “A simple routine is to take calcium supplements with meals, and to take zinc away from calcium-rich meals or supplements,” said Lal. It should be noted that while certain forms of calcium, like calcium citrate, can be taken with or without food, other forms, like calcium carbonate, should be taken with meals.

What happens: Copper and zinc compete for absorption in your small intestine. “These minerals compete for absorption as they share similar transporters in their absorption pathways,” Helen Tieu, RD, told Health.

Why it matters: While copper and zinc compete for absorption, research suggests that issues typically stem from taking very high doses of zinc. Studies show that long-term intake of 60 milligrams or more of zinc per day can impair copper absorption and reduce blood copper levels. Copper deficiency can cause health issues such as anemia and muscle weakness. It’s important to stay below the Tolerable Upper Intake Level (UL) for zinc, which is set at 40 milligrams per day for adults.

What to do instead:  “Space the dosing of these minerals throughout the day, or opt for a balanced mineral or multivitamin supplement that contains lower doses of zinc and copper,” recommended Tieu.

What happens: Platelets are small blood cells that help your blood clot to prevent bleeding. “Ginkgo biloba and high-dose fish oil both inhibit platelet function, so taking them together can raise bleeding risk due to thinning of the blood,” explained Lal. This is especially important for people taking blood-thinning medications, like Warfarin. “The same caution applies to other blood-thinning supplements like garlic, ginger, turmeric, and ginseng,” he said.

Why it matters: Taking high doses of supplements that have blood-thinning properties can be dangerous, especially for people already on blood thinners. It’s also not a good idea to take blood-thinning supplements before a scheduled surgery, as this could increase bleeding risk.

What to do instead: To keep yourself safe, Lal recommended avoiding stacking these supplements, considering lower or alternate dosing, and reviewing your regimen with your doctor, especially prior to any surgical procedure.

What happens: Activated charcoal is a popular natural remedy for digestive ailments, such as bloating, as well as for eliminating toxins from the body. However, activated charcoal should not be taken with vitamins and mineral supplements. “Activated charcoal will bind to minerals and vitamins, therefore impacting absorption,” Lal said.

Why it matters: Taking activated charcoal for prolonged periods can prevent the absorption of essential nutrients, leading to deficiencies and health issues. It can also interfere with medication absorption, reducing their effectiveness, which can be dangerous.

What to do instead: Lal recommended using binders like activated charcoal in short, targeted doses and separating them from all medications and supplements for 2 to 4 hours.

Dietary supplements may seem harmless, but taking too much of certain supplements or misusing them can result in reduced nutrient absorption and other serious side effects. 

“It’s important to consider supplement interactions since many share absorption pathways and alter enzymes that process and clear medications,“ explained Warren. “These interactions can lead to dangerous complications, including failure of effectiveness of medications, nutritional deficiencies, and other side effects.”

Warren advises speaking with your medical provider if you’re unsure of a specific supplement combination or have questions about supplement safety. 

Here are a few tips to help you stay safe when taking supplements:

• Space them out: Take competing nutrients a few hours apart. For example, take iron in the morning and calcium later in the day.
• Avoid large doses: Unless specifically recommended by a healthcare provider, it’s best to stay below the established upper limits (ULs) for vitamins and minerals to prevent excessive intake, which may deplete other vital nutrients in some cases.
• Pair wisely: Some supplement combinations help absorption, like vitamin C and iron, while others, like calcium and iron, hinder absorption.
• Talk with your healthcare provider: If you have questions or need advice on supplements, speak with a trusted healthcare provider.