Thanks for doing this study! I'm glad that people are doing RCTs on creatine with more subjects. (Also, I didn't know that vegetarians had similar amounts of brain creatine as omnivores, which meant I would've incorrectly guessed that vegetarians benefit more than omnivores from creatine supplementation). 

Here's the abstract of the paper summarizing the key results and methodology:

Background

Creatine is an organic compound that facilitates the recycling of energy-providing adenosine triphosphate (ATP) in muscle and brain tissue. It is a safe, well-studied supplement for strength training. Previous studies have shown that supplementation increases brain creatine levels, which might increase cognitive performance. The results of studies that have tested cognitive performance differ greatly, possibly due to different populations, supplementation regimens, and cognitive tasks. This is the largest study on the effect of creatine supplementation on cognitive performance to date.

Methods

Our trial was preregistered, cross-over, double-blind, placebo-controlled, and randomised, with daily supplementation of 5 g for 6 weeks each. We tested participants on Raven’s Advanced Progressive Matrices (RAPM) and on the Backward Digit Span (BDS). In addition, we included eight exploratory cognitive tests. About half of our 123 participants were vegetarians and half were omnivores.

Results

Bayesian evidence supported a small beneficial effect of creatine. The creatine effect bordered significance for BDS (p = 0.064, η²_P = 0.029) but not RAPM (p = 0.327, η²_P = 0.008). There was no indication that creatine improved the performance of our exploratory cognitive tasks. Side effects were reported significantly more often for creatine than for placebo supplementation (p = 0.002, RR = 4.25). Vegetarians did not benefit more from creatine than omnivores.

Conclusions

Our study, in combination with the literature, implies that creatine might have a small beneficial effect. Larger studies are needed to confirm or rule out this effect. Given the safety and broad availability of creatine, this is well worth investigating; a small effect could have large benefits when scaled over time and over many people.

Note that the effect size is quite small:

We found Bayesian evidence for a small beneficial effect of creatine on cognition for both tasks. Cohen’s d based on the estimated marginal means of the creatine and placebo scores was 0.09 for RAPM and 0.17 for BDS. If these were IQ tests, the increase in raw scores would mean 1 and 2.5 IQ points. The preregistered frequentist analysis of RAPM and BDS found no significant effect at p < 0.05 (two-tailed), although the effect bordered significance for BDS.

Awesome work! I'd be interested in seeing a similar study done on nicotine. It's hard to sift through the bias against it due to its association with tobacco use. I find consistent creatine use, along with nicotine and caffeine during intense work sprints, to be great.