Probiotics have long been considered foundational to a healthy routine, but despite their lengthy history as a mainstream supplement, there’s still plenty of confusion about these itty-bitty gut bugs. Perpetual questions around which strains to choose, how many billions of bacteria to look for, and whether or not to refrigerate them can make taking probiotics feel as mind-boggling as trying to remember high school calculus.

If you could use some clarification on the puzzle of probiotics, read on. This expert-approved guide has the a’s to your microbiota q’s.

When to Take Your Probiotics 

There’s lots of chatter out there surrounding the best time of day to send probiotics down the hatch, and whether to do so with food. So are probiotics pals with breakfast, lunch, or dinner—or none of the above?

Turns out, time of day isn’t as important as whether you take them on an empty belly. “Probiotics have a better survival rate on an empty stomach or right before a meal, when stomach acid levels are lower,” says The Vitamin Shoppe dietitian Brittany Michels, M.S., R.D.N, L.D.N. “Avoid taking them after a meal.” The high amounts of stomach acid present then are bad news for probiotics’ survival.

How Many CFU to Choose

The epic numbers of microbes contained in probiotic supplements are enough to make your head spin. With such a wide range, you might be tempted to close your eyes and snag any old bottle, pin-the-tail-on-the-donkey-style. But according to the Cleveland Clinic, a general recommendation for adults is to choose a product with at least one billion colony-forming units (CFU).

Just how many billions, though, depends on your personal needs. “Probiotic depletion may be influenced by many factors, including but not limited to a lack of fermented foods in the diet, unmanaged stress, processed foods, alcohol, added sugar, inflammation, food intolerances, medication or antibiotic usage, and digestive disorders,” explains Michels. “The more depleting factors you have, the more support you need.” 

Read More: Which Type of Probiotic Supplement Is Right for You?

Generally, daily supplements that contain between 15 and 50 billion CFU are common, according to Michels. Most people can start anywhere within this ballpark range. High billion counts (like up to 400 billion), meanwhile, are usually reserved for individuals needing serious gut rehab support (for example, post-antibiotic treatment). If you’re not sure exactly how many billions of bugs might benefit your gut, check with your doctor or a dietitian.

Consider Your Specific Health Concerns

Not all probiotics are ideal for every health goal. “Probiotics are best taken when specific strains are used for specific concerns,” says Dr. Chris Damman, M.D., Clinical Associate Professor of Gastroenterology & Medicine at the University of Washington. For example, if you’re looking to support immunity, you might try Lactobacillus paracasei, Lactobacillus casei, and Lactobacillus fermentium. One study found that people who took these strains for 12 weeks saw significant respiratory health and immunity benefits. Other research, meanwhile, has identified Saccharomyces boulardii as a potentially helpful strain for those experiencing chronic gut health symptoms, so look for that if it’s regularity you’re after. 

Before hitting “add to cart,” you want to make sure you’re clear on your motives for taking a probiotic in the first place—and that the product you’re eyeing is specifically formulated to support those goals. Most supplement labels and e-commerce platforms offer basic information on a product’s health benefits. 

Where to Store Them

Probiotics have a reputation for being rather temperamental when it comes to temperature. So is it best to stash them in the fridge, or can they hang out with the beans and pasta in your pantry?

Read More: What’s the Deal With Refrigerated Probiotics? 

In general, you can keep probiotics under whatever temperature conditions you purchased them (i.e. refrigerate them if you bought them refrigerated). That said, it’s always a good idea to check a product’s packaging for storage instructions. 

And if you’re wondering whether you should pick a shelf-stable or refrigerated supplement, know this: Neither one is superior. While it might seem that refrigerated products are more likely to retain their potency than shelf-stable options, there’s plenty of technology nowadays that protects the bacteria in shelf-stable products from heat, light, and oxygen, says dietitian Wendy Lord, R.D., a contributor to the health professional network Health Reporter.

Which you choose really comes down to personal preference (are you likely to forget your probiotic if it’s tucked away in the fridge?) and whether your healthcare provider has recommended you take certain strains in order to support your unique health needs and goals, Lord says.

The Importance of Quality

As with every supplement, don’t forget to look for indicators of third-party testing. Approval labels from companies like NSF, Consumer Labs, and USP are a sign of quality and purity of ingredients. 

Another sign of quality: diversity. “The gut flora is diverse, so supplements should have a variety of bacterial strains as well.”

Lastly, make sure you don’t get more than you bargained for if you’re allergic to certain foods or dyes. “Check the ingredient list to confirm absence of allergens, fillers, artificial colors, or preservatives,” Michels recommends.