Aside from similar-sounding names, both creatine and creatinine are produced at a relatively constant rate in our bodies.
But, what sets them apart? Which one's the good guy, and which one's a villain?
We took a deep dive into chemistry and a pile of scientific articles to get to the bottom of the issue.
But before we get into differences, let's get started with the basics of creatine and creatinine.
What You Should Know About Creatine and Creatinine
Creatine is an amino acid that is made primarily in the liver.
It is then transported to your muscles, where it is used for energy and muscle activity.
As an amino acid, creatine can do great things for your body, while creatinine is a waste product that occurs due to creatine breakdown in muscle cells.
Both creatine and creatinine are present in the blood and urine and tell us a lot about the overall kidney condition.
What is Creatine?
Creatine is an organic acid that is produced naturally in your body from amino acids.
These acids get converted into creatine phosphate and phosphocreatine, stored in the skeletal muscles, and used as an energy source.
Creatine phosphate, a natural compound commonly found in vertebrates, helps make adenosine triphosphate (ATP), a compound that powers muscle contraction necessary for explosive strength and high-intensity exercise.
The Role of Creatine
Creatine can be your best workout buddy that fuels your weight-lifting session or high-intensity performance.
"Between 1.5 and 2 percent of the body's creatine store is converted for use each day by the liver, the kidneys, and the pancreas. It is transported through the bloodstream and used by parts of the body that have high energy demands'', -Debra Rose Willson, Ph.D., healthlilne.com.
P.S. If you're a vegan, these creatine brands are right for you.
What is Creatinine?
Creatinine is a waste product of muscle breakdown and protein metabolism in which creatine gets converted to creatinine in a non-enzymatic manner. 
Creatinine production may vary depending on the intake of creatine, muscle mass, body mass index, and diet. In particular, cooked red meat or fish can affect creatinine concentration.
The Role of Creatinine
Creatinine, unlike creatine, doesn't play any vital role in the human body, but it can help establish your kidneys' health through a series of blood checks.
Since most blood creatinine is filtered by the kidneys and released into the urine, the blood creatinine levels may indicate how well the kidneys are working.
But remember that creatinine will likely be higher in muscular people in comparison to others.
What Is a Creatinine Test?
Science has evolved over the past century and resulted in a couple of different ways to assess kidney function.
The measure of serum creatinine may be used to gauge how quickly your kidneys filter blood (estimated glomerular filtration rate). Since serum creatinine levels may vary from one person to another, the estimated GFR may offer a clearer picture of kidney function.
Creatinine is freely filtered through the glomerulus until it's finally excreted in the urine. A decrease in the estimated GFR affects the filtration, resulting in increased levels of creatinine.
There is a difference between examining creatinine in your urine (creatinine clearance test) and looking at creatinine in your blood (serum creatinine - SCr).
The elevated serum creatinine level may indicate either chronic kidney disease or acute nephritis, but it could also be due to certain medications (such as cyclosporine).
The Relationship Between Creatine Supplements and Renal Function
Some scientists argue that creatine can cause kidney injury, but recent studies show no negative effect when taken within recommended doses. 
Although in the past, there have been some cases suggesting that creatine might cause kidney injury in people with kidney disorders, according to research, creatine doesn't seem to cause harm in healthy people.
‘’In fact, evidence suggests that creatine is one of the world's most tested supplements and has an outstanding safety profile.’’ - Rudy Mawer, MSc, CISSN.
Many hard-training athletes and bodybuilders can vouch for its efficiency and a number of benefits, while very few side effects have been reported, among which body weight gain is the only confirmed one.
Contrary to popular belief, creatine doesn’t seem to cause dehydration or muscle cramping.
If you experience unusual muscle cramps, pain, or weakness, your doctor may order a creatine kinase (CK) test.
Creatine kinase is an enzyme found in the skeletal muscles, brain, and heart.
An elevated level of this enzyme may indicate some heart diseases or conditions that produce muscle damage.
Does Creatine Use Affect the Level of Creatinine?
Creatinine levels depend on different factors, and taking a creatine-based supplement can be one of them.
Another thing that makes a difference is the form of creatine supplement you're taking.
For example, one study found that the use of creatine monohydrate caused only a marginal increase in serum creatinine .
Other evidence suggests that the creatine supplements, when combined with a high protein diet, may increase serum creatinine concentration in your blood, but this alone doesn't mean that your renal function is at risk.
A series of different blood tests may be required to determine if these levels indicate kidney damage or not. Just skip that juicy meat meal before testing.
And remember that people with more muscle mass also have elevated creatinine levels, even if they're not taking supplements.
So, What's the Difference Between Creatine and Creatinine?
The relationship status of creatine and creatinine seems complicated, but it boils down to a couple of crucial points.
There's that hero that boosts your muscles, creatine, and its less glamorous twin -a by-product - creatinine.
While creatine products can somewhat influence creatinine concentration in your blood, there are other forces at play.
For example, science confirms the correlation between muscle mass and increased creatinine level.
Have you checked your creatinine recently? How did it go? Tell us about it in the comments.