That's a good question, and is something you really should know when using these cards (especially if you're using a Vortex half). I learnt by playing.
The calculations for this ability DO NOT INCLUDE THE FREE PILL.
0 pillz -> 1 returned (this is a special case)
1 pill -> 1 returned
2 pillz-> 2 returned
3 pillz -> 2 returned (hence the ability defeat: recover 2/3 pillz)
4 pillz -> 3 returned
5 pillz -> 4 returned
6 pillz -> 4 returned
7 pillz -> 5 returned
8 pillz -> 6 returned
9 pillz -> 6 returned
You get the idea. You get a return on all your pillz except every third pill.
When I joined the game it automatically played with the free pill showing, so that's how I learnt and still play. So if you are like me, just remember the free pill is not included in the calculations. If you play without it showing, you obviously don't need to worry about this.
I don't use Recover Pillz cards much but as far as I can tell, the pillz regained is just pillz used times (2/3) rounded down with a min of 1.
(Pillz regained= Pillz Used x (2/3) for the mathematicians)
Pillz Used: Pillz Regained
and so on.
Its not OP. All recover pills are DEFEAT type. As long as you make enemy eat significant damage and dont give easy positions to get pills back this ability is nullified.
Recover pills is mostly about damage control; people use them so they can recover from bad decisions. But it can be used to deliver guaranteed double-nukes too where opponents countering ability becomes totally nullified, but you need to get upper hand first...