Creatine a one of the most common supplements used to increase muscle mass and boost performance, is a nitrogen based organic acid naturally existing in mammals. In the body It can be converted into phosphocreatine to provide energy for muscle and nerve tissues. Creatine and its analog, cyclocreatine, have been considered cancer suppressive metabolites due to their effects on suppression of subcutaneous cancer growth.
Recently, however its been emerging that supplementation on Creatine was demonstrated to promote cancer metastasis. Various mouse models, used in the lab have revealed that creatine promoted invasion and metastasis of pancreatic cancer, colorectal cancer, and breast cancer.
Thus, creatine possesses considerably complicated roles in cancer progression.
A recent article in 2021 showed that dietary supplementation with creating might increase metastatic cancer growth by fueling the cancer cell through certain protein enzymes called GATM that are found in the cell.
Increasing the function of the GATM enzyme and increasing the de novo formation of creatine was found to enhance liver metastasis and shorten the life span of rats. 
Another theory as to why creatine might be cancerogenic is the fact that it might enhance the production of HCAs ( heterocyclic amines). These compounds, which are usually produced from cooking or charring meat or other foods, have been implicated in the promotion of cancer, especially lung, stomach, bladder, colon and breast cancer.
“HCAs are among the most potent mutagens assessed by the Ames Salmonella test (Ames et al. 1975), providing biological plausibility for association studies.”
Although a major exposure to PhIP, 8-MeIQx, IFP and 4,8-DiMeIQx is through the consumption of cooked meats, these mutagens have also been detected in other products such like processed foods, beverages such as beer and wine, and cigarette smoke 
In a previous study where meat based patties were exposed to 6min at 300ºC the levels of HCAs, such as MeIQx, Iso-MeIQx and PhIP were found to have increased in urine after the consumption of 275 g of cooked minced beef patties (containing roughly 1.1 g of creatine).
Conventional creatine supplementation regimens, up to 20 g day may be less ‘harmful’ than a 275 g grilled meat in terms of HCA formation, And adding to this further studies supporting the safety of creatine found that, supplementing pigs with up to 50 g day for 5 days did not increase HCA formation upon frying of pork meat.
Although the study shows there might be an increased risk, in healthy subjects it was concluded that creatine supplementation did not seem to increase the risk of cancer. 
So Is creatine supplementation safe?
When used as a supplement at appropriate doses, creatine is likely safe to take for up to five years.
Of course as with any dietary supplement, it’s important to choose a product that follows a recommended manufacturing practice, and is tested both inhouse and by 3rd party to ensure the product’s quality. So I don’t recommend going to buy the shady products down at the alley store.
The major side effect of Creatine you might experience is Weight gain, generally as lean body mass (If used in small quantities of course)
Creatine might be unsafe for people with preexisting kidney problems. However, further research is as with all food supplements is need.
- Wyss M, Kaddurah-Daouk R. Creatine and creatinine metabolism. Physiol Rev. 2000 Jul;80(3):1107-213. doi: 10.1152/physrev.2000.80.3.1107. PMID: 10893433.
- Pereira RT, Dörr FA, Pinto E, et al. Can creatine supplementation form carcinogenic heterocyclic amines in humans?. J Physiol. 2015;593(17):3959-3971. doi:10.1113/JP270861