"Sarcosine is formed from dietary intake of choline and from the metabolism of methionine, and is rapidly degraded to glycine, which, in addition to its importance as a constituent of protein, plays a significant role in various physiological processes as a prime metabolic source of components of living cells such as glutathione, creatine, purines and serine."
Sugar doesn't interfere with creatine levels.
And you can't take it on clozapine, as it is dangerous, unlike sugar which wouldn't be...
I imagine that some people don't get much effect from taking it and hence what you have heard, however, clinical studies have proven different. It would take time and careful management of taking it to have any measurable effect on a patient, management that people don't adhere to.