No less an authority than Mythbusters tested it out a few years ago. Yes, they actually put a Christmas tree in a base with some Viagra-spiked water. The trial also included some other homespun tree preservatives: Sprite, aspirin, and bleach. They tried a couple of topical treatments as well, spraying one tree with hairspray and another with urethane.
In the final results, bleach tied with Viagra for keeping the tree from losing its needles. Unfortunately, the trees turned weird colors and looked sickly. Hairspray turned out to be the winner for preventing needle loss while preserving a healthy green color. Thing is, they couldn't really recommend spraying your Christmas tree with flammable chemicals.
So, if you have a holiday tree, the best thing you can do is start with a fresh cut at the bottom, put it in plain fresh water, and make sure the water level never drops below the cut surface, or it will seal over.
If you still want to celebrate the season with Viagra, trim your tenebaum with some of these festive Viagra Christmas ornaments.