This gem causes any attacked enemy (regardless of the Proc Coefficient of the attack, skill or spell) to suffer Poison damage over time. This effect is renewed by further attacks, but does not stack.

How good is gem of efficacious toxin?

Gem of Efficacious Toxin is +10% damage & -10% damage by enemies. The rank based poison damage will eventually become irrelevant, except for speed runs. Bane of the Stricken at rank=150 is +2.3% damage.

Does gem of efficacious toxin work on follower?

If you are wearing BotT, it will work on any slowed target regardless of who slowed them, be it yourself or your follower or other party member. That's just how the gem works, not exactly follower proccing it.