The Poison Pill Defense, Explained
The question
Explain how a poison pill (shareholder rights plan) actually works mechanically: what triggers it and what happens to the acquiring shareholder's stake.
General educational practice only. This is not an actual, confidential, leaked, or firm-provided interview question. Check important technical details against primary learning materials.
The answer
A poison pill works through dilution. The board adopts a rights plan that sets a trigger threshold, typically between 10 and 20 percent of the company’s shares. If any single holder crosses that line without board approval, every other shareholder immediately receives the right to buy additional shares at a steep discount.
That massively dilutes the hostile acquirer’s economic stake and voting power, making it economically irrational to cross the threshold. The board can adopt the pill without a shareholder vote and can waive it at any time if it decides to negotiate, so a bidder who wants to get past the pill either needs the board’s consent or must first replace the board through a proxy fight.
Mechanically, it is not about making shares more expensive; it is a dilution time bomb that destroys the acquirer’s position.
Accretion / dilution
| Acquirer standalone net income | 300 |
| + Target net income | 80 |
| + After-tax synergies | 15 |
| − After-tax incremental interest | (10) |
| Pro forma combined net income | 385 |
| Acquirer standalone EPS | $3.00 |
| Pro forma share count | 125 |
| Pro forma EPS | $3.08 |
| Accretion | 2.7% |
Also asked as
- List, in escalating order, the steps a hostile bidder typically takes after a private acquisition approach is rejected by a target's board.
- A target agrees to a $2,800M sale with a $98M breakup fee. Is this fee within the typical market range? Show the percentage.
- Explain the difference between a staggered (classified) board and a poison pill as takeover defenses, and why the combination of the two is historically considered especially effective against hostile bidders.
- What are the Unocal and Revlon standards in Delaware corporate law, and how do they constrain a target board's ability to reject a hostile bid or resist a topping bid after signing a friendly deal?
- Distinguish a standard (target-paid) breakup fee from a reverse breakup fee: who pays each, what circumstances typically trigger each, and why reverse breakup fees are often larger as a percentage of deal value.
- A target has 80M shares outstanding and adopts a poison pill with a 12% trigger. A hostile bidder crosses the trigger by acquiring 12.5% of shares. If the pill causes all non-triggering shareholders' share count to increase by 80% via discounted purchases, compute the bidder's diluted ownership percentage after the pill triggers, and explain why the bidder would rationally avoid crossing the trigger in the first place absent a board waiver.
- A signed merger agreement has a $150M target-paid breakup fee on a $5,000M deal, no go-shop, and a 5-business-day matching right for the original acquirer. A competing bidder emerges 30 days after signing with a fully financed offer of $5,400M. Walk through the board's fiduciary obligations under the no-shop's fiduciary out, compute the net proceeds to target shareholders if the original acquirer does not match and the board switches, and explain what feature of this deal-protection package is most exposed to a Revlon-based legal challenge.
- An acquirer proposes a reverse breakup fee of 9% of deal value tied specifically to antitrust-clearance failure on a deal with real regulatory risk, versus a rival bidder in the same auction offering only a 3% RBF for the identical risk. Explain what each RBF size signals about the respective bidders' confidence in clearance, and describe a scenario where offering the larger RBF would actually be a value-destructive decision for the acquirer offering it.
- A target board is considering deploying greenmail against a hostile bidder who has accumulated a 12% stake, versus instead adopting a 'just in time' poison pill. Explain why greenmail is now rarely used compared to decades ago, and walk through why a board can often adopt an effective poison pill within days of a bear hug letter even if it has no standing pill in place beforehand.
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