In humans, intravenous administration of S-adenosylmethionine (100 mg, 500 mg, and 0.5 mg/kg of body weight) directly into the bloodstream has a half-life of approximately 80 to 100 minutes.[1][2] Furthermore, there are detectable increases in S-adenosylmethionine concentrations in the blood and the cerebrospinal fluid after oral, intravenous, and intramuscular administration, suggesting that S-adenosylmethionine crosses the blood-brain barrier.[3]
In humans, intramuscular administration of S-adenosylmethionine (0.5 mg/kg of body weight) shows approximately 80–90% bioavailability — i.e., 80–90% of S-adenosylmethionine injected into a muscle appears in the blood.[1] However, when orally administered in humans, the bioavailability of S-adenosylmethionine is poorer,[3][4] as low as 2–3% in some studies.[5] However, some studies show that enteric-coated capsules of S-adenosylmethionine, which are protected from degradation by stomach acid, have improved bioavailability in humans when compared to uncoated S-adenosylmethionine.[6][7] Novel formulations such as phytate salts[8] and solid lipid nanoparticles[9] have further improved oral bioavailability in rodents, but these formulations remain to be tested in humans.