When a company receives an unsolicited bid it doesn't want to accept, its board doesn't just say no and hope the acquirer goes away. It reaches for a specific toolkit of legal and financial defenses built up over decades of M&A practice — and understanding how each one actually works, mechanically, is a recurring theme in M&A interviews.

What Is a Poison Pill (Shareholder Rights Plan)?

A poison pill, formally a shareholder rights plan, is the most common first line of defense against a hostile bid. The board adopts a plan that grants all shareholders — except the acquirer — the right to buy additional shares at a steep discount once the acquirer's stake crosses a set trigger threshold, typically 10-20%. Because the acquirer is deliberately excluded from that right, everyone else's shareholding grows while the acquirer's stays fixed, which mechanically shrinks the acquirer's percentage ownership and voting power.

The pill rarely gets triggered in practice. Its real function is deterrence: it makes continuing to accumulate shares on the open market prohibitively expensive, which pushes the acquirer toward the negotiating table instead. The board can typically redeem the pill at any time once it's satisfied with the terms on offer.

How a Poison Pill Actually Dilutes an Acquirer

The dilution math is where a lot of candidates lose the thread in interviews — reciting "it dilutes the acquirer" without being able to show the arithmetic. If an acquirer holds a 12% toehold stake and the rights plan lets every other shareholder double their holdings at a discount, the acquirer's ownership doesn't just shrink a little — it can fall by nearly half, from 12% to roughly 6%, without the acquirer selling a single share. That's a large enough hit to voting power and economic claim that most acquirers back off open-market accumulation and start talking to the board instead.

We walk through this calculation step by step, with real figures, in Hostile Takeover and Defense Tactics, which also prices a competing white knight offer against the original hostile bid.

Other Common Takeover Defenses

The poison pill is usually paired with other structural and tactical defenses:

  • Staggered (classified) board: only a fraction of directors — often one-third — are up for election each year, so even a successful proxy fight can't hand an acquirer board control in a single cycle. It doesn't stop a takeover, but it stretches the timeline and cost enormously.
  • White knight: the target's board recruits a friendlier third-party acquirer to make a higher, more palatable offer, effectively replacing the original hostile bidder.
  • Pac-Man defense: a rare, aggressive tactic where the target turns around and launches its own tender offer for the hostile acquirer — only realistic when the target has comparable size and financial firepower.
  • Golden parachute: generous severance packages for existing executives triggered by a change of control, which raises the acquirer's effective cost of the deal.

Why Boards Don't Always Deploy These Defenses

None of this is unconditional. Delaware courts, under the Unocal and Revlon lines of cases, require a board's defensive response to be proportionate to the actual threat and consistent with its fiduciary duty to shareholders. A pill or staggered board deployed reflexively, without genuinely engaging with the bid, can expose directors to litigation and shareholder backlash — which is why boards typically pair these defenses with real negotiation rather than a blanket refusal to talk.

See the Defense Mechanics Worked Through With Numbers

If you want to practice the calculation rather than just the theory, Hostile Takeover and Defense Tactics walks through a full poison pill dilution calculation and a white knight offer comparison, with a model answer and follow-up questions on staggered boards and Pac-Man defenses. For the M&A consideration side of a deal, see Cash vs. Stock Consideration.