Is Hydrogen Water Safe? 

Are there any side effects?

In the USA, it has been officially recognized at the federal level that adding hydrogen to beverages and drinking water is absolutely safe and permissible.

This decision was made by the Food and Drug Administration (FDA) on November 28, 2014, under GRAS Notice No. 520.

In response to a query from the American firm MitoGene Beverage Company, LLC about whether hydrogen can be added to drinking water and beverages, the FDA replied that based on scientific data, it has no objections or questions regarding this matter and hydrogen is recognized as safe - Generally Recognized as Safe (GRAS).

https://www.cfsanappsexternal.fda.gov/scripts/fdcc/?set=GRASNotices&sort=GRN_No&order=DESC&startrow=1&type=basic&search=Hydrogen