Agmatine [4-(aminobutyl)guanidine], an endogenous neurotransmitter , is decarboxylated L-arginine and is a metabolic product of mammalian cells. Agmatine binds to α2-adrenergic receptor, it induces the release of some peptide hormones, and was reported to inhibit nitric oxide synthase (nitric oxide synthases are responsible for the production of nitric oxide from L-arginine) [1,2].
Agmatine Benefits for Exercise
There are many claimed benefits of agmatine and a lot of disinformation is circling all over internet. Some even claim that agmatine increases nitric oxide production via inhibition of nitric oxide synthase enzyme, which is nonsense. A study by Elena Galea and others  reported that agmatine is a competitive inhibitor of all nitric oxide synthase isoenzymes. However, it seems that agmatine stimulates some types of nitric oxide synthase while inhibiting others. Morrissey and Klahr  reported that agmatine caused activation of nitric oxide synthase in endothelial cells (layer of cells on inner surface of blood vessels). Agmatine appears to act directly on endothelial cells to increase the synthesis of nitric oxide, which induces them to cause vasodilatation .
Treatment of Chronic Pain
Agmatine has potential to treat chronic pain, addictive states and brain injury due to its ability to inhibit hyperalgesia (increased sensitivity to pain) and its neuroprotective activity . Therefore it may potentiate the effects of analgesics used during recovery from injury. Feyza Aricioglu and Hale Altunbas  reported modulatory effect of agmatine on anxiety and depression. Accumulating evidence suggests that agmatine possesses nootropic effects and that is a novel neurotransmitter . More and more evidence is accumulating on the involvement of agmatine in the processes of learning and memory, however, direct evidence of its involvement is currently lacking .
In vitro studies are also showing insulin-like effect . Sener A. et al.  have shown that agmatine causes stimulation of insulin secretion but fails to affect insulin release in the absence of glucose. Activation of imidazoline receptors by agmatine in adrenal glands lowered plasma glucose through beta-endorphin release in diabetic rats lacking insulin [9,10].
Possible Agmatine Side Effects
Currently there are no known side-effects and there is limited evidence to draw any conclusions.
(Other common names: 1 Amino 4 guanidinobutane, 4-(Aminobutyl)guanidine, Argmatine, AGmass™, AgmaMax®, AgmaPure®, 1-(4-Aminobutyl)guanidine)
Feng, Yangzheng, John E. Piletz, and Michael H. Leblanc. “Agmatine suppresses nitric oxide production and attenuates hypoxic-ischemic brain injury in neonatal rats.” Pediatric research 52.4 (2002): 606-611.
Galea, Elena, et al. “Inhibition of mammalian nitric oxide synthases by agmatine, an endogenous polyamine formed by decarboxylation of arginine.” Biochem. J 316 (1996): 247-249.
Morrissey, Jeremiah J., and Saulo Klahr. “Agmatine activation of nitric oxide synthase in endothelial cells.” Proceedings of the Association of American Physicians 109.1 (1997): 51-57.
Reis, Donald J., and Soundararajan Regunathan. “Is agmatine a novel neurotransmitter in brain?.” Trends in pharmacological sciences 21.5 (2000): 187-193.
Aricioglu, Feyza, and Hale Altunbas. “Is agmatine an endogenous anxiolytic/antidepressant agent?.” Annals of the New York Academy of sciences 1009.1 (2003): 136-140.
Liu, Ping, et al. “Spatial learning results in elevated argmatine levels in the rat brain.” Hippocampus 18.11 (2008): 1094-1098.
Weitzel, Günther, Brigitte Pfeiffer, and Wieland Stock. “Insulin-like partial effects of agmatine derivatives in adipocytes.” Hoppe-Seyler´ s Zeitschrift für physiologische Chemie 361.1 (1980): 51-60.
Sener, Abdullah, et al. “Stimulus-secretion coupling of arginine-induced insulin release: Insulinotro action of argmatine.” Biochemical pharmacology 38.2 (1989): 327-330.
Chang, Chin-Hong, et al. “Increase of β-endorphin secretion by agmatine is induced by activation of imidazoline I 2A receptors in adrenal gland of rats.” Neuroscience letters 468.3 (2010): 297-299.
Hwang, S-L., et al. “Activation of imidazoline receptors in adrenal gland to lower plasma glucose in streptozotocin-induced diabetic rats.” Diabetologia 48.4 (2005): 767-775.