Although you can take your multivitamin with any beverage you like, it isn't always advisable. Some beverages, including coffee, contain substances that could interfere with the absorption of some of the nutrients in your vitamin. It's better to drink your coffee about 15 minutes before or a few hours after you take your vitamin.
Effect on Overall Absorption
The caffeine in coffee will speed up the digestive process by increasing the contractions that force food through your digestive tract. When your multivitamin is moved more quickly through the digestive process, there will be less time for you to absorb the vitamins and minerals it contains, and some may pass through your body without being absorbed.
Reduces Iron Absorption
Like tea, green leafy vegetables and beans, coffee contains tannins. If your multivitamin contains iron, tannins can decrease its absorption. Tannins interfere with the absorption of nonheme iron, which is the type found in plant-based foods and supplements, including multivitamins that contain iron. Drinking your coffee separately from when you take your multivitamin will help minimize this effect. Take your vitamin with orange juice or another vitamin C-rich juice to increase iron absorption.
May Limit Calcium Absorption
If you drink more than 2 or 3 cups of coffee per day, the caffeine in the coffee may reduce the amount of calcium you absorb and increase the amount of calcium you excrete. However, this will only cause issues with your bone density if your overall calcium consumption is low, according to the Institute of Medicine. The Office of Dietary Supplements notes that each cup of coffee can cause you to lose 2 to 3 milligrams of calcium.
Effect on Thiamine Absorption
Minerals aren't the only micronutrients affected by coffee. The tannins in coffee can turn the thiamine in your multivitamin into a form your body has difficulty absorbing, according to MedlinePlus. Getting plenty of vitamin C in your diet seems to prevent this problem, which appears to be a risk mainly for people with low thiamine consumption. Most Americans get plenty of thiamine in their diet from foods such as whole grains, enriched or fortified grain products, lentils, pecans and peas.