When you’re a writer, work can be fun, but it can also be tiring. When tired, writers—like many professionals—have a tendency to turn to caffeine. Seriously, I’ve downed an entire french press’ worth of joe just writing this lede paragraph.

Sports can also be fun. Sports can also be tiring. Can a little coffee or a cold cola help you be better at sports?

The answer is, well, it depends. Both on who you ask and what sport you’re talking about.

In 2000, Jack Hartley from the Vanderbilt Psychology Department broke down three main theories behind caffeine’s effects on athletes and discussed their validity:

Caffeine can help improve endurance.

Many studies related to caffeine’s effects on athletes have proven inconclusive, but this one is pretty clearly true. Hartley cites a 1998 study by T.E. Graham, in which distance runners participated in double blind placebo tests to find out just how much caffeine would help. Graham found that athletes who took caffeine pills one hour before exercising were able to run an extra 7.5-10 minutes at 85 percent of maximum O2 consumption compared to the control groups.

Per Hartley, one theory for why caffeine helps improve stamina is that the caffeine forces your body to burn more fat and fewer carbohydrates. “Glycogen is the principal fuel for muscles, but fat is the most abundant resource that the body uses for energy. Caffeine enters the body and forces the working muscles to utilize as much fat as possible. This delays the immediate depletion of glycogen.” When your body first burns fat instead of glycogen, it saves some of the good stuff for later, letting you work your muscles harder for longer.

Other studies have found similar results, including one test where cyclists managed to generate 3.5 percent more power during a 24-mile time trial.

All in all, this thesis appears true, which is great news if you’re participating in endurance sports like cycling or running.

Caffeine can improve your focus and reactions.

This one seems like it should be true, and it is, although results on how that can best be applied have been inconsistent.

Here’s what we know: caffeine is a proven stimulant to the central nervous system, often targeting the medulla and cortex and even affecting the spinal cord if taken in large doses. Studies have found that caffeine intake can give you a greater ability to concentrate for 1-3 hours, which helps for quick thinking and rapid reactions—useful for actions like hitting a baseball or tennis ball.

Caffeine can provide a needed burst for athletes like sprinters and swimmers.

As it turns out, this seems to be where folks most often go wrong. Studies have indicated caffeine actually has little effect on athletes requiring a quick burst of energy. The lone exception that may exist: some tests suggest caffeine can help to strengthen muscle contractions. So, if you’re a weight lifter, caffeine might be able to give you a little boost, although studies don’t appear to be conclusive on that front.

Should you, weekend warrior hoping to shave 30 seconds off your 5K time, turn to caffeine for a boost? For starters, not without talking to your doctor. Hartley points out that most doctors do not support the use of caffeine to boost athletic performance because it can have negative health effects likes sleep deprivation, nausea, cramping, anxiety, headaches, and gastrointestinal instability. If you are going to use caffeine, it’s important to know your limits—if you’re a regular coffee drinker anyway, a cup an hour before a long run shouldn’t cause much harm. But if you’ve never had so much as a Diet Coke, it may be best to avoid using caffeine to boost performance, and if you do, you should ease into it.

Hartley does have two tips for caffeine use, though. For starters, the ideal time to ingest caffeine is 2-3 hours prior to exercise, as studies have found it takes several hours for caffeine to enter the body and exploit the use of fat. Also, if you have a big competition coming up, refrain from caffeine use for 3-4 days beforehand. This lets your tolerance levels for caffeine drop, thus increasing its effects. But again, this should all be done carefully and ideally with the approval of your doctor.