“Multivitamins are like an insurance policy,” says Singh. As he explains it, in an ideal world we would be able to obtain the minimum required nutrients from our diet in the form of food. But, as you know, this is not always possible. At times when we’re lacking in nutrients (like we’re traveling, grabbing granola for lunch, or just can’t make the effort to whip up some leafy greens after work), multivitamins can help fill in any gaps.

The purpose of this is to make sure you don’t develop a deficiency in any one nutrient, which is becoming increasingly common. According to the CDC Second nutrition report1, just under 10 percent of the US population was malnourished. This depends on many factors, such as age, gender and ethnicity, and the number can be as high as 30 percent in certain populations. The report found that the most common deficiencies were vitamin B6, iron and vitamin D — all of which are commonly found in multivitamins.

If you’re ready to hit buy multivitamins, there’s one thing you need to know first: not all are created equal. “I try to find multivitamins from a high-quality company with a good reputation,” says Singh. Some Green Flags use organic ingredients, natural flavors and colors, and products that are free of fillers, dyes and gluten, he said.

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When you find a high-quality product, you may wonder how long it will take before you know it works.

There are no hard and fast rules, but experts recommend waiting a few weeks to a few months before expecting changes. And even then, Singh says, you’re unlikely to notice anything too drastic (but that doesn’t mean your supplement isn’t doing its job).

That’s because multivitamins are designed to provide small amounts of a variety of nutrients to maintain healthy levels, not to actually correct a deficiency. If you have a true deficiency, multivitamins likely do not contain enough of this nutrient to correct it.

For example, if you are deficient in vitamin D, doctors recommend taking at least 5,000 IU of vitamin D per day to correct the deficiency, not just to maintain healthy levels. why Because, according to certified clinical nutritionist Lindsey Boyers, you need 1,000 IU of vitamin D3 per day to raise your levels2 by about 10 ng/mL4, but a typical multivitamin contains only about 400 IU of vitamin D.

However, there are some signs that your multivitamin is working: