There are 3 different kinds of Lazarus Pits. Two are purely science-based, but contain different chemicals; One has a mixture of 20 different toxins while the other has a unique chemical Ra's calls "Lazarium". Both function the same and require a massive electrical pulse applied to the pit. The only difference between the two is that the Lazarium based version is able to revive and revitalize a corpse that has been dead for less than 2 months, while the artificial pit can only restore a near-death subject back to full health and youth. The negative effects are the same, but are weaker in the Lazarium pit due to its purity.
The third type is magic based. It requires pure Lazarium, but no electrical pulses. Instead, Ra's uses magical artifacts and relics to induce an energy spike in the pit, which can charge it for days. Because it's magical, it can revive those who have been long dead for centuries. Ra's used it once, having been dead for months and a loyal follower dumped him in the pit. The side effects are way worse due to the dark magic involved, as it causes one to actually hear the voices of the dead and damned for a whole month, or in some cases, a whole year, driving the user mad after a while. The only one to successfully use it and keep his sanity was Ra's himself.