Sarcosine and D-Serine are two endogenous substances that are currently being researched for a number of conditions. Although these substances have many interesting properties and may be effective add-on treatments to disorders such as depression and schizophrenia, they are too nonspecific to work effectively as a standalone, and should never be used in place of a doctor’s recommendation. That being said, there is some promising evidence for a nootropic, antidepressant and antipsychotic effect.
D-serine vs L-serine
This article includes references from studies using both enantiomers of serine. The two may have somewhat similar effects since L-serine is converted to D-serine by serine racemase, an enzyme that requires vitamin B6 to function. It is thus important to supplement vitamin B6 when supplementing L-serine.
Studies involving both forms of serine clearly demonstrate its interesting scope of effect. That being said, L-serine may be less effective since it requires an extra step before it can be used by the body.
Mood and Depression
D-serine produces both acute and chronic antidepressant effects in animal models. Chronic use was also reported to lower markers of anxiety. Studies have also shown that it may prevent cognitive impairment in subjects exposed to stressful situations (like a job interview) but not in normal day-to-day situations. D-serine, therefore, does not strictly meet the definition of a nootropic. It is, however, ubiquitous in the body, and with many interesting effects in promoting cerebral health, certainly a worthy supplement.
Sarcosine was the star of a landmark 2013 study, which underpinned a role of the Glycine transporter 1 (GlyT1) in depression. The antidepressant effect of sarcosine and other GlyT1 inhibitors may be related to the increased levels of glycine, an inhibitory neurotransmitter that seems to have an anti-stress effect. The study found sarcosine monotherapy to be superior to the SSRI citalopram (Celexa) in remitting depression, with rapid onset of action and few side-effects. The role of glycine in mental disorder may be much larger than originally thought.
As expected, users have reported varying levels of effectiveness from sarcosine as an antidepressant, with some drawing a comparison between ketamine or citalopram, while others have not noticed a significant effect. The research has suggested a strong general effect, which may still nevertheless depend on the individual.
Schizophrenia and Psychosis
The glutamate system is the largest excitatory system in the brain and it commands functions across every region, so one would expect sarcosine to be useful in depression, memory loss, or interestingly, psychotic disorders. The glycine transporter’s influence over NMDA receptors (which are essential for memory and learning) may also explain why sarcosine makes a useful adjunctive therapy to antipsychotics. It’s all to do with the involvement of NMDA and glutamate in schizophrenia:
… the hypofunction hypothesis of glutamatergic neurotransmission via N-methyl-D-aspartate (NMDA) receptors in the pathophysiology of schizophrenia suggests that increasing NMDA receptor function via pharmacological manipulation could provide a new therapeutic strategy for schizophrenia. The glycine modulatory site on NMDA receptor complex is the one of the most attractive therapeutic targets for schizophrenia. […] Some clinical studies have demonstrated that the GlyT-1 inhibitor sarcosine (N-methylglycine) shows antipsychotic activity in patients with schizophrenia. Currently, a number of pharmaceutical companies have been developing novel and selective GlyT-1 inhibitors for the treatment of schizophrenia..
Because of the diverse actions of the glycine transporter and its involvement in other neurotransmitter systems, sarcosine has also been found to modulate striatal dopamine release. This may help further explain its antipsychotic properties.
While it may not be terribly effective on its own, used in conjunction with pharmaceuticals, sarcosine can deliver a real knockout blow, conveniently complimenting pharmaceuticals in treating both negative and positive symptoms. This is particularly remarkable because most pharmaceuticals were developed to address only positive symptoms; there has been a race to find more effective treatments for the negative symptoms, which have often come in the form of over-the-counter supplements, exercise programs, and specific diets.
Sarcosine and N-acetylcysteine were both found to restore glutamate activity in mGluR5 knockout mice Low activity of this receptor may be related to schizophrenia. Interestingly, positive allosteric modulators of mGluR5 may have nootropic properties. 
Sarcosine is known to have potential in Parkinson’s and other dopamine dysfunction disorders, such as ADHD. Sarcosine and its metabolite glycine modulate dopamine in the striatum, which includes the hippocampus.
Another study has confirmed as helpful the addition of sarcosine to antipsychotic therapy, concluding it increases survival of neurons and glial cells in the dorsolateral prefrontal cortex (a key structure damaged by aging and drug abuse, it is involved in executive functions, such as working memory, cognitive flexibility, planning, inhibition, and abstract reasoning). Sarcosine may soon become a popular supplement in the psychiatric industry.
Even though they are far from being popular nootropic supplements, D-Serine and Sarcosine are two fascinating compounds.
If you are being medicated for schizophrenia or depression, you should definitely consider supplementing one of these two compounds to augment your psychiatric medication. Sarcosine, in particular, is very inexpensive and has a lot of evidence backing up its use.  
Nootropic users who have tried Sarcosine report improvements in focus, motivation and visual acuity, and there is some evidence that it may help those living with ADHD.
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