A popular supplement among athletes, creatine is considered relatively safe and may also help with neurodegenerative diseases like muscular dystrophy and Parkinson's and Huntington's diseases, according to the Cleveland Clinic. But if you have high blood pressure, be careful: Creatine may not be safe.

Creatine is a natural source of energy in the body, produced by your kidneys, liver and pancreas and stored primarily in your skeletal muscles, per the Cleveland Clinic. When you call on your muscles during physical activity, they call up the reserves. Your heart, brain and other organs also use small amounts of creatine to function properly.

Any excess creatine, according to the Cleveland Clinic, turns into creatinine, which passes out of your body through your urine — which means that every day your body must make creatine anew. You can get some extra creatine from eating red meat and seafood, according to the U.S. National Library of Medicine (NLM), but if you're vegetarian or an athlete who wants to improve your upper- and lower-body strength, you might be considering taking creatine supplements.

Athletes of all levels rely on creatine supplements, though they're most popular among athletes in power sports like football, wrestling and bodybuilding, according to the Cleveland Clinic. Creatine is permitted in professional sports, as well as by the International Olympic Committee and the National Collegiate Athletic Association.

If you have a lower level of creatine when you start taking supplements, you're more likely to notice a benefit than those who have higher initial levels because there's a saturation point — your skeletal muscle will only hold so much creatine, according to the NLM. Increasing intake can't raise levels beyond that saturation point, which you can reach with what's called a loading dose within your first few days of supplementing.

So, if you can take creatine for athletic performance or age-related muscle loss, how much do you need? There's no clear answer to that, but according to the NLM, most athletes start with a loading dose for a short time — typically 20 grams for 4 to 7 days — before switching to maintenance doses, which are generally 2 to 10 grams daily. If you're interested in creatine for muscle strength, the loading dose is similar (20 grams a day for 5 to 7 days), with a maintenance dose of 1 to 27 grams a day.

Pay close attention to your dosage. According to the Mayo Clinic, high doses of creatine can be potentially unsafe and have unpleasant side effects like muscle cramps, nausea, stomach pain and diarrhea, dizziness, dehydration, water retention, heat intolerance and fever. You also may gain weight.

Moreover, many medications — including nonsteroidal anti-inflammatories such as ibuprofen (Advil, Motrin) and naproxen sodium (Aleve) — do not mix well with creatine supplements, per the Mayo Clinic, increasing risk of damage to your kidneys. Another no-no when you take creatine is caffeine, which may erase any effectiveness of the supplement, according to the Mayo Clinic.

Regarding the effect of supplementation on blood pressure, a November 2015 study of creatine loading phase in college-aged males in the​International Journal of Food and Nutritional Science​ found that creatine supplements had no adverse effects on their blood pressure. "However, this is a group that did not have hypertension to begin with," says New York City-based gastroenterologist Niket Sonpal, MD. "Thus, the concern is mainly for patients who already have high blood pressure."

Dr. Sonpal explains why creatine and blood pressure don't mix in some people: "Creatine makes your kidneys work harder, [so] there is a risk that it may cause your blood pressure to rise and/or worsen. The data is still conflicting as to a true link." But, to be safe, he says, "the best thing to do is avoid this supplement if you have hypertension."

"No matter how healthy you are," Dr. Sonpal adds, "always let your doctor know before you take creatine or any other supplement."