Re: VOY: The Eternal Tide by Kirsten Beyer Review Thread (Spoilers!)
The only time I can remember them actually using the last one after Season 1 was in "Demon", and even then it was a silly contrived plot device.
They ran out of deuterium, somehow.
We are all the sum of our tears. Too little and the ground is not fertile and nothing can grow there; too much, the best of us is washed away.