Spoiling Baccano is lame so...
Szilard only had half the knowledge needed to create the elixir after he ate Maiza's brother. So he spent the next few centuries trying to figure out the rest.
The first elixir that Dallas and his goons were given was incomplete. They couldn't be killed but they'd die from old age. The final elixir Miria and the rest drank was the ultimate, perfected elixir.