Thanks for the comment, Michael. I did not time myself, but 7.5 s/d is my guess for the difference between taking 5 capsules (including one of creatine; capsules are 23.5 $/year cheaper under my assumptions) and the 4 I already take after lunch (D3, Omega 3, iodine and a multivitamin; I take B12 before breakfast). I have all the supplements in the same place, so it would not take me much time to take another capsule.

Thanks, Yanni. My hypothesis was also that vegetarians and vegans would benefit more from creatine supplementation, but it looks like there is no clear evidence for that. Below is the abstract of the systematic review of Kaviani et al. (2020) (emphasis mine).

Background: Creatine monohydrate is a nutritional supplement often consumed by athletes in anaerobic sports. Creatine is naturally found in most meat products; therefore, vegetarians have reduced creatine stores and may benefit from supplementation. Objective: to determine the effects of creatine supplementation on vegetarians. Data sources: PubMed and SPORTDiscus. Eligibility criteria: Randomized controlled trials (parallel group, cross-over studies) or prospective studies. Participants: Vegetarians. Intervention: Creatine supplementation. Study appraisal and synthesis: A total of 64 records were identified, and eleven full-text articles (covering nine studies) were included in this systematic review. Results: Creatine supplementation in vegetarians increased total creatine, creatine, and phosphocreatine concentrations in vastus lateralis and gastrocnemius muscle, plasma, and red blood cells, often to levels greater than omnivores. Creatine supplementation had no effect on brain levels of phosphocreatine. Creatine supplementation increased lean tissue mass, type II fiber area, insulin-like growth factor-1, muscular strength, muscular endurance, Wingate mean power output, and brain function (memory and intelligence) in vegetarian participants. Studies were mixed on whether creatine supplementation improved exercise performance in vegetarians to a greater extent compared to omnivores. Limitations: Studies that were reviewed had moderate–high risk of bias. Conclusions: Overall, it appears vegetarian athletes are likely to benefit from creatine supplementation.

The 2nd sentence I highlighted above refers to "exercise performance", but it is also unclear whether vegetarians and vegans benefit more from creatine supplementation with respect to cognitive function. From my post:

Sandkühler et al. (2023) “found no indication that our vegetarian participants benefited more from creatine than our omnivore participants (in fact, the creatine effect was smaller in vegetarians than omnivores to a non-statistically significant extent). This is in line with Solis et al. [22, 24] who did not find a difference in brain creatine content between omnivores and vegetarians”.

Thanks, Oscar. I do not know how much creatinine one should have in the blood to maximise the cognitive benefits. From the meta-analysis of Xu et al. (2024), it looks like the optimal supplementation is an open question (emphasis mine):

Despite these limitations, the results of this study provide promising evidence for creatine as a cognitive enhancer, particularly in improving memory and information processing speed. Notably, these findings specifically support creatine monohydrate as an effective form of supplementation. This evidence offers a scientific basis for the application of creatine in cognitive enhancement and provides direction for future research. Future studies should aim to optimize creatine supplementation strategies, including exploring the optimal dosage, supplementation duration, and long-term effects, to maximize its cognitive benefits. Additionally, further research is needed to elucidate the mechanisms by which creatine affects cognitive function and to investigate its interactions with other cognitive interventions, such as cognitive training and other nutritional supplements.

It is still very early days. It is not even clear whether creatine supplementation improves overall cognitive function. Below is the forest plot of the 6 studies meta-analysed in Xu et al. (2024).

Below is a scatter plot with the effect size and daily dosage of the above studies. A positive value favours creatine. I think the legend of the horizontal axis above should have [Con] on the left, and [Cre] (creatine) on the right.

There is basically no correlation. The coefficient of determination is 0.961 % (and the slope is negligibly negative, -0.00906 d/g).

In terms of how much creatine one can take, from the systemic review of Kreider et al. (2017):

[...] There is no compelling scientific evidence that the short- or long-term use of creatine monohydrate (up to 30 g/day for 5 years) has any detrimental effects on otherwise healthy individuals or among clinical populations who may benefit from creatine supplementation.

So I assume it is fine for one to experiment with daily dosages higher than the conventional 5 g/d.

One can determine the optimal daily dosage speculating about the function describing the benefits. For my assumption that the benefits are proportional to the logarithm of 1 plus the daily dosage in g/d, the benefits in $/year are 10.6*10^3/ln(1 + 3)*ln(1 + "daily dosage in g/d") = 7.65*10^3*ln(1 + "daily dosage in g/d"). The cost in $/year is "cost in $/year/(g/d)"*"daily dosagr in g/d". So the net benefits in $/year are 7.65*10^3*ln(1 + "daily dosage in g/d") - "cost in $/year/(g/d)"*"daily dosage in g/d". The derivative of this with respect to the daily dosage is 7.65*10^3/(1 + "daily dosage in g/d") - "cost in $/year/(g/d)", which is 0 for a daily dosage in g/d of 7.65*10^3/"cost in $/year/(g/d)" - 1. This is the dosage for which the net benefits are maximum because their 2nd derivative with respect to the daily dosage, -7.65*10^3/(1 + "daily dosage in g/d")^2, is always negative. For my cost of 31.7 $/year/(g/d) (= 95.2/3), the optimum daily dosage is 240 g/d (= 7.65*10^3/31.7 - 1). I think this is way too high because I am underestimating the diminishing returns of the benefits. My function implies they grow indefinitely with ln(1 + "daily dosage in g/d"), but they will eventually stagnate, decrease and become negative (harmful) for sufficiently large daily dosages, even if my assumption is a good approximation for small dosages.