Results from four treatment trials not covered in these meta-analyses have been recently published. Prior to this, the majority of glutamatergic treatment trials have used naturally occurring agents, including the glycine transport inhibitor sarcosine. Glycine transport inhibitors, in general, increase brain glycine levels by blocking its removal from the synaptic space, although studies with sarcosine have not yet been performed. Sarcosine was shown to be more efficacious than D-serine in a recent head-to-head trial [25], but, because of safety concerns [26], is not available for use in the USA.
