Is there a difference between NADH and NADH2?
They are often used interchangeably to indicate the reduced form of $\ce{NAD+}$.  The overall reaction when oxidizing some molecule $\ce{RH2}$ is: $\ce{RH2 + NAD+ -&gt; NADH + H+ + R}$.  The proper reduced $\ce{NAD+}$ is $\ce{NADH}$ (it accepts two electrons and one proton), but sometimes $\ce{NADH2}$ is used to account for that second hydrogen that gets removed from the substrate being oxidized.  The notation $\ce{NADH2}$ doesn't really take into account the fact that the second hydrogen is charged, and not bound to the $\ce{NAD}$ in the same way that the first hydrogen is, so it is confusing.  The notation: "$\ce{NADH + H+}$" is more correct and is also sometimes used.