There are more than 90,000 vitamin and dietary supplement products sold in the US. They come in pills, powders, drinks, and bars. And they all anticipate some better versions of ourselves—selves with sturdier bones, slimmer waist lines, heftier muscles, happier intestines, better sex lives, and more potent noggins. They foretell of diseases dodged and aging outrun.

On the whole, we believe them. Supplements are a $30 billion industry in the US. Recent surveys suggest that 52 percent of Americans take at least one supplement—and 10 percent take four or more. But should we? Are we healthier, smarter, stronger, or in any way better off because of these daily doses?

The answer is likely no. Most supplements have little to no data to suggest that they’re effective, let alone safe. They’re often backed by tenuous studies in rodents and petri dishes or tiny batches of people. And the industry is rife with hype and wishful thinking—even the evidence for multivitamins isn’t solid. There are also outright deadly scams. What’s more, the industry operates with virtually no oversight.

The Food and Drug Administration is not authorized to review supplements for safety and efficacy before they hit the market. That means the bottles and capsules that line the aisles of grocery stores, pharmacies, and specialty shops may not contain what they say they do. They may have wildly variable amounts of supplements in each batch. They may have hidden, illicit components like amphetamines. They may even have harmful contaminants such as lead and cadmium.

And even after these potentially dangerous drugs hit the market, the FDA lacks the capacity to keep up with all the products and health claims in order to flush out bad actors. It’s hard to blame them. Doctors, too, have trouble keeping up with the 90,000 plus products.

To help out, two experts at Harvard have put together a quick guide of effective supplements recently. Preventative medicine experts JoAnn Manson and Shari Bassuk lay out which ones are actually effective and when. It’s a relatively short list. Here's what they and other experts recommend, along with a quick crash course for navigating the world of supplements.

What to take

First, the pair emphasizes: the best way to get vitamins and minerals is through food in a healthy diet. Eaten in food, micronutrients are often better absorbed into the body and present in optimal ratios. That’s in contrast to gulping a hunk of concentrated compounds in a pill.

For healthy, midlife to older adults

Vitamin B 12 —if you’re over 50. At this age, you may no longer be able to adequately absorb the protein-bound form of vitamin B 12 from food. A supplement can help make up the difference to get you to the recommended daily dose of 2.4 μg per day.

Vitamin D—As we age, our skin and kidneys become less efficient at making and processing vitamin D, respectively. For some, a vitamin D supplement may help them get to the current recommended daily intakes of 600 IU (international units) per day for adults up to age 70 and 800 IU per day for those older. Some health organizations recommend higher levels, up to 2,000 IU per day, but this is currently controversial. If too much vitamin D builds up in the blood, it can become toxic, causing nausea, weakness, and constipation.

Calcium—maybe. Calcium absorption also decreases with age. Current recommended doses for calcium are: 1,000mg per day for men 51 to 70; and 1,200mg per day for women 51 to 70 as well as all adults older than 70 years. Recent analysis suggested that moderate doses (less than 1,000mg per day) may help reduce the risk of bone fractures and bone mass density loss in men and post-menopausal women aged 65 or older. But, too much calcium can cause constipation. Too much from supplements, specifically, may also increase the risk of kidney stones.

Overall, multivitamins are not recommended for the generally healthy. There’s just no clear evidence to prove that they have any benefit. A 2013 analysis of 27 trials involving more than 400,000 people concluded that there was no clear evidence that they reduce cardiovascular disease, cancer risks, or reduce all-cause mortality. A study tracking nearly 6,000 men for 12 years found they didn't protect or improve cognitive function. One large, randomized, double-blind, placebo-controlled US trial in men found that some who took multivitamins had a slight reduction in total cancer risk (particularly those with previous nutritional deficiencies and cancer diagnoses). But the trial needs to be repeated with women and more baseline data of nutrition status.

For those with conditions

Various medications, dietary restrictions, and specific conditions can throw off what nutrients you take in and absorb. For instance, people with untreated celiac disease may not get enough iron, magnesium, zinc, B vitamins, and vitamin D. Long-term use of a proton pump inhibitor, such as Prilosec for stomach acid, may interfere with absorption of vitamin B 12 , calcium, and magnesium over time. Those taking metformin for type II diabetes may have malabsorption of vitamin B 12 , though the evidence has been inconsistent on this. And some patients with age-related macular degeneration may benefit with a specialized supplement containing vitamins C, E, beta-carotene, and zinc with copper, according to one large-scale, randomized, controlled trial.

If you’re pregnant

“The evidence is clear,” Manson and Bassuk write, that women who may become pregnant and pregnant women in their first trimester should get adequate folic acid (0.4 to 0.8mg per day). Based on epidemiological data, this nutrient helps prevent defects in an embryo’s neural tube, the precursor for the brain and spinal cord. It’s still not clear, however, how folate does this. Researchers have noted that it’s involved in a complex network of metabolic reactions linked to neural tube closure.

Though folic acid is found in many foods, it’s one of the few nutrients that is actually more bioavailable in the synthetic forms found in supplements and fortified foods.

There’s evidence from several small trials that calcium supplementation during pregnancy may reduce the risk of gestational hypertension and preeclampsia. But larger trials are needed to shore up this finding.

There’s little data on whether taking omega 3 fatty acids, vitamin D, and multivitamins during pregnancy substantially improves maternal and child health.

For healthy kids and infants

Like healthy adults, healthy kids don’t need a multivitamin and should avoid high doses of individual micronutrients. Recently, some research has suggested that omega 3 fatty acids may complement treatment for attention-deficit/hyperactivity disorder and autism, but there is not enough data to draw any conclusions.

The American Academy of Pediatrics, as well as the Centers for Disease Control and Prevention, recommend that breastfed infants get vitamin D supplements. The AAP also recommends iron supplements for breastfed infants. Breast milk simply doesn’t contain enough of these nutrients for babies. Babies on formula or eating solid foods, however, typically get enough of these from their diet.