Molecular hydrogen is generally considered nontoxic, even at high doses, and side effects have rarely been reported.[1] In one study using hydrogen-rich water, a few participants experienced loose stools, heartburn, and headaches; however, there was no placebo group to compare to.[2] In patients inhaling hydrogen gas for several hours daily, drowsiness and agitation have been reported.[1]
There are some additional risks depending on the administration method. Hydrogen gas at concentrations above 4% can be explosive when mixed with oxygen, and injectable hydrogen-rich saline raises the risk of infection and injury. Furthermore, these dosage forms generally require specialized equipment and medical supervision.[3][4]