I read with interest the editorial in December 2019 on ‘A possible role for sarcosine in the management of schizophrenia’. Professor David Curtis did suggest that ‘it seems to be universally well tolerated with an absence of significant side-effects’.^{Reference Curtis1} I wonder if addition of sarcosine to medication for schizophrenia is actually safe for every patient. It is well accepted that sarcosine level increases in many cases of carcinoma of the prostate gland. Indeed, it may well be a marker for carcinoma of the prostate.^{Reference Kdadra, Höckner, Leung, Kremer and Schiffer2–Reference Cernei, Heger, Gumulec, Zitka, Masarik and Babula4} It is thought that this elevated level of sarcosine is produced by the prostatic cancer cells. This does not mean that it causes the cancer. However, there are at least two important studies in the literature that comment on this issue. Sreekumar et al^{Reference Sreekumar, Poisson, Rajendiran, Khan, Cao and Yu5} in Nature in 2009 found metabolomic profiles delineating a potential role for sarcosine in prostatic cancer progression and Khan et al^{Reference Khan, Rajendiran, Ateeq, Asangani, Athanikar and Yocum6} in Neoplasia in 2013 found increased alteration of benign prostatic epithelial cells upon the addition of sarcosine to prostatic cells. I wondered therefore if a note of caution should be sounded about the use of sarcosine supplement in older men with schizophrenia, especially those with signs of prostatic hypertrophy. Prostate cancer is the most common cancer in males over 70 and the second most common cause of cancer deaths in men.