Use our checklist to see how well you’re doing.

If you take one or more dietary supplements, you may assume that all you need to do is swallow them with a sip of water. But that’s not true.

To maximize the health benefits and reduce the risk for side effects, most supplements should be taken in certain ways and/or at certain times.

For example, it’s best to take multivitamins and fish oil with meals. This helps you avoid stomach upset when taking multivitamins, and the fats included in the average meal enhance the absorption of fish oil. What you need to know about other commonly used supplements…*

• Vitamin C. Vitamin C is among the most potent antioxidants. People take it to boost immunity, maintain healthy blood pressure and reduce the risk for heart disease.

Main mistake: Taking it all at once. Splitting vitamin C into two daily doses improves absorption and keeps the blood level of the vitamin elevated all day, rather than the “peak” and “valley” of blood levels that occurs with once-a-day dosing. In addition, some people may experience diarrhea and/or stomach pain if they take a large amount (such as 1,000 mg) at one time.

What to do: To prevent gastrointestinal upset and improve the benefits, divide the dose if you take more than 500 mg of vitamin C per day. For example, if you usually take 1,000 mg a day, take 500 mg in the morning and a second 500-mg dose in the afternoon or evening.

Many people are able to tolerate 1,000 mg or more at one time. However, don’t exceed the tolerable upper intake level of 2,000 mg daily without consulting your doctor.

Also helpful: Buffered forms of vitamin C, which contain sodium ascorbate or calcium ascorbate, are less likely to cause stomach upset than unbuffered forms.

Vitamin C is also less likely to cause stomach upset and diarrhea if taken with food. Note: Chewable ascorbic acid can damage tooth enamel.

• B vitamins. Many people take a B-complex supplement because B vitamins can boost energy and mood.

Main mistake: Taking B vitamins at night, which can cause insomnia for some people.

What to do: Take B vitamins in the morning. B vitamins sometimes cause mild stomach upset on an empty stomach, so take them with food.

Also helpful: Ask your doctor about taking magnesium when you take a B-complex supplement. This mineral tempers the vitamins’ energizing effects and helps prevent insomnia or anxiety.

• Vitamin E. This fat-soluble vitamin is an important antioxidant.

Main mistake: Taking it without food. Like all of the fat-soluble nutrients, including vitamins A, D and K, vitamin E requires the presence of fat for optimal absorption. You’ll absorb less if you take it without food.

What to do: Take vitamin E with meals. The fat that is naturally present in the average meal is all you need to absorb the vitamin.

Helpful: Get plenty (approximately 500 mg) of vitamin C—either in your diet or with a supplement—if you’re taking vitamin E supplements. The nutrients recharge each other so that they stay active for longer periods.

Also important: Take a vitamin E supplement that contains mixed tocopherols (all four of the forms of vitamin E found naturally in food). Many vitamin E products on the market contain only alpha-tocopherol. Taking alpha-tocopherol by itself can deplete gamma-tocopherol, a form of vitamin E that appears to protect against both heart disease and prostate cancer. Studies have shown that taking 400 IU per day of alpha-tocopherol by itself increases the risk for heart failure and prostate cancer. These adverse effects may be due to depletion of gamma-tocopherol. In a typical 400-IU dose of mixed-tocopherol vitamin E, look for 50 mg to 100 mg of gamma-tocopherol. This approximates the ratio that normally is found in foods.

• Calcium. Millions of Americans use supplemental calcium to strengthen the bones and prevent osteoporosis.

Main mistake: Taking calcium between meals, which may increase the risk of developing kidney stones

What to do: When calcium is taken with meals, it blocks the absorption of oxalate, a compound in some foods that contributes to the formation of the most common type of kidney stone (calcium oxalate). Research suggests that taking calcium with meals reduces the risk for kidney stones by decreasing the amount of oxalate in the urine. In contrast, taking calcium between meals increases urinary calcium levels without lowering the amount of oxalate in the urine; the net result is an increase in stone risk.

The recommended dietary allowance of calcium (from food and supplements) is 1,200 mg daily for women age 51 and older and men age 71 and older…and 1,000 mg daily for men ages 51 to 70.

Also helpful: If you take calcium supplements, ask your doctor about taking magnesium with them. In some cases, supplemental calcium is believed to deplete magnesium from the body. And low magnesium could explain in part why, in a recent study, people who took calcium had a 30% higher risk for heart attacks than those who took placebos.

It’s also a good idea to take 800 IU to 1,200 IU of vitamin D daily. Like calcium, vitamin D helps prevent bone loss. Vitamin D also helps reduce the risk for falls in older adults. To enhance absorption, take vitamin D with a meal that has some fat.

• Iron. If you have iron-deficiency anemia, you need extra iron. Your doctor will probably recommend 325 mg of ferrous sulfate daily. Once the anemia is corrected, you should ask your doctor if you need to continue to take iron to prevent anemia.

Main mistake: Taking iron at the same time as vitamin E. Each nutrient is blocked by the other, so you’ll absorb lower levels of each one.

What to do: Don’t take iron with vitamin E. Large doses of some minerals also may interfere with iron absorption, so don’t take a high-potency multiple mineral at the same time as iron. However, a low-potency mineral supplement, such as 10 mg of zinc, 100 mg of calcium and 100 mg of magnesium, should not interfere with iron absorption.

When using iron, wait two to three hours before taking vitamin E alone or in a multivitamin supplement. Also helpful: Take iron with 100 mg or more of vitamin C (from food or a supplement). This increases the amount of iron that’s absorbed.

Important: I don’t recommend supplemental iron unless you’ve received a blood test and are actually deficient. There is some evidence, though conflicting, that people with normal levels of iron who take iron supplements might be at increased risk of developing diabetes or heart disease.

• Glucosamine. A study in Archives of Internal Medicine that looked at 202 patients with arthritis of the knee found that those taking glucosamine sulfate had less disease progression than those taking placebos. Glucosamine, a supplement made from chitin (pronounced kite-m), a substance in the shells of shrimp and other shellfish, also can reduce joint inflammation and pain.

Main mistake: Taking glucosamine hydrochloride. It’s less effective than glucosamine sulfate, possibly because the sulfur in glucosamine sulfate provides additional benefit.

What to do: Take 1,500 mg of glucosamine sulfate daily—all at once or in divided doses any time of day. There is no definitive research showing that combining glucosamine sulfate with chondroitin sulfate, a compound that’s naturally present in cartilage, increases the effectiveness, although chondroitin sulfate also appears to be helpful for people with osteoarthritis. Caution: Glucosamine can increase intraocular pressure, so people with glaucoma or elevated intraocular pressure should talk to their doctors before taking glucosamine.

*If you have a chronic medical condition or take any kind of medication, check with your doctor before taking supplements. For example, people with kidney disease or severe heart disease must be supervised by a physician when taking magnesium. Also ask whether the supplements can be taken at the same times as medication.