There are a slew of potential reasons your energy levels might be flagging—from lack of sleep and poor diet to depression, anemia, stress, thyroid issues, and even lack of exercise. And sometimes caffeine just doesn’t do the trick (not to mention that drinking too much can cause ill effects from increased heart rate to dizziness and anxiety).

So what about vitamins and supplements to help boost your energy? There are a few that experts swear by.

But first, a few caveats.

“If someone is dealing with low energy, my initial response would be to recommend a series of tests before considering supplements,” Dr. Elizabeth Sharp, internist and medical director at Health Meets Wellness in New York City, tells Fortune. “Testing for vitamin deficiencies, such as vitamin D and vitamin B12, especially in vegetarians, is crucial. Additionally, I would screen for thyroid disorders if other symptoms suggest such a condition, and I would also rule out anemia and iron deficiency. It’s important to identify the underlying cause accurately to ensure appropriate and effective treatment.”

Once they are determined to be appropriate, she says, be cautious when making your purchase. “Be wary of supplements that are far cheaper than the rest or look like a good deal,” Jolene Brighten, naturopathic endocrinologist and author of Is This Normal?, warns. “Often these are using poor quality ingredients and in some cases, because they are not regulated, they won’t actually have what the label lists.” (While the U.S. Food and Drug Administration does technically regulate supplements, it doesn’t approve them for safety or effectiveness before they are sold to the public.)

To help avoid such risks, suggests Cathi Dennehy, a doctor of pharmacy, dietary supplement researcher, and professor at the University of California San Francisco, start by speaking with your health care provider or a registered dietitian who is well-versed in supplements. Then, when you’re ready to buy, be sure to do some research, especially on the brand.

Consumers may want to consult the National Institutes of Health [Office of Dietary Supplements] as well as subscription-based sources, such as Consumer Lab, which charges a fee for access to its test results, which looks into issues ranging from possible contaminants to making sure a supplement contains what it claims to. “A question that I get asked fairly frequently is, ‘Is this brand that I’m taking a good brand?’” says Dennehy, who points to these sources for answers.

Finally, she says, when deciding which vitamins and supplements could help with energy: “This is my take on it: If you’re deficient in it, then correcting can be helpful. But if you’re not deficient in it, then it’s not likely to be helpful.”

All of that said, below are four favorites to consider.

1.Vitamin B12

A nutrient that helps keep our body’s blood and nerve cells healthy and while also helping to make DNA, the genetic material in all of our cells, vitamin B12 also helps prevent megaloblastic anemia, a blood condition that makes people tired and weak, according to the NIH Office of Dietary Supplements. It’s why vitamin B12 supplements are often a solid go-to for people lacking energy, as long as there’s evidence of a deficiency.

“There are certain vitamins that we know to be more likely to be associated with deficiency in the general population. And about 15% of the population is deficient in B12. So that’s something that you could test for,” says Dennehy. She adds that certain populations are more likely to be deficient—including vegans, because B12 is bound to animal protein in food; folks with an autoimmune disorder called pernicious anemia, who are unable to absorb the vitamin; and people who take a lot of over-the-counter stomach-acid reducers, like famotidine, because they don’t have the stomach acid to release the B12 from the animal protein.

Additionally, warns Brighten, “be cautious taking these in the late afternoon or evening since they can interfere with sleep in some people.”

And, once again, says Dennehy, “the only reason it would improve energy would be if you’re correcting a deficiency.”

2.Vitamin D

According to the Cleveland Clinic, some of the top symptoms of vitamin D deficiency in adults are fatigue, muscle weakness, and low mood, which can lead to feeling down and exhausted. So does it make sense that supplementing vitamin D could add some pep in your step?

Possibly. A study published in the journal Medicine compared the results of two groups with fatigue and vitamin D deficiency—one of which was given vitamin D supplements, the other a placebo. In just four weeks, the group given vitamin D experienced significant improvements in energy levels.

“It’s very hard to get the vitamin D you need from your diet; oily fish and fortified dairy products are the only important sources,” notes a Harvard newsletter on supplements. So supplements do make good sense for most adults.

“Vitamin D is beneficial during the winter,” says Sharp, “with 1,000 to 2,000 IU daily being safe for most people.” Brighten adds that “it is also important to be getting enough sunlight” and suggests testing D levels to determine what you need.

3.Creatine

Creatine is a compound made in your liver, kidneys, and pancreas and found naturally in foods such as red meat and fish. It’s primarily stored in your muscles in the form of phosphocreatine, which is “the initial energy source for all exercise and the preferred source for ‘explosive’ movements such as lifting heavy objects, jumping, and short sprints. As such, creatine monohydrate supplements are widely used to increase strength performance,” according to the Department of Defense Dietary Supplement Resource.

And unlike some other supplements, says Dennehy, “that one actually has some decent evidence for being beneficial for someone who does, let’s say, high-intensity exercise.” The International Society of Sports Nutrition, she adds, has a favorable stance on creatine, as does the Natural Medicines Database. “It seems to improve that maximal, intense-exercise output.” So creatine supplements—available in powder, capsule, or gummy form—makes sense, she says, “if you’re someone who’s doing a lot of heavy athletic activity.”

Brighten agrees, noting, “Creatine is a very well-studied supplement that offers both brain and muscle health benefits. In addition, it can help your cells generate a molecule called ATP, which is often referred to as the energy currency of the cell. Creatine can help improve your workouts, which can result in you feeling more energetic and getting better sleep.”

4.Iron

“Again, it would make sense if you’re deficient in iron,” says Dennehy. “And like the World Health Organization notes, iron deficiency is common worldwide. About 30% of the world has a more severe form of iron deficiency anemia, and we know iron deficiency anemia can result in tiredness and fatigue and weakness, and decreased immunity. So this is where you would want to have your kind of lab work done.”

Because excess iron, on the other hand, can be toxic to the body, she says.

Brighten adds, “Outside of patients who are regularly menstruating, pregnant, or who have confirmed iron deficiency anemia, we don’t typically recommend iron supplementation because it can have negative effects on your health.”

“It can cause gastrointestinal issues if not needed,” warns Sharp.

Honorable mentions

Finally, two bonus supplements to consider include beetroot powder, which Brighten says “is rich in nitrates, which the body converts into nitric oxide, which leads to increased blood flow and delivering oxygen to your tissues, which can result in more energy,” and magnesium. That, she says, “is a mineral that a lot of people benefit from and can help with energy levels by improving sleep. Additionally, magnesium is required for the metabolism of foods, which is how we obtain energy, and in regulating our blood sugar, which helps us maintain energy.”