Definition
A poison pill (formally, a shareholder rights plan) is a defensive measure in which the target’s board issues “rights” to existing shareholders that, upon a triggering event (typically a person crossing a defined ownership threshold like 10–20%), entitle holders other than the triggering person to buy shares at a steep discount.
Why it works
Activation makes it economically prohibitive for the bidder to acquire control because the bidder’s stake is massively diluted. The pill effectively forces the bidder to negotiate with the board.
Modern dynamics
- Most U.S. public companies don’t have a pill on the shelf permanently but maintain the ability to adopt one quickly (“morning-after pill”)
- Pills are typically adopted in response to an actual or threatened bid
- Bidders rarely trigger pills; instead they sue, run a proxy contest to remove the board, or accept the need to negotiate
Trigger threshold
10–20% ownership is the typical trigger for traditional pills. Modern variants designed for activist defense often trigger lower (5–7.5%) and/or include “wolfpack” provisions targeting coordinated holders.