Poison Pill
A poison pill, or shareholder rights plan, is a defence against hostile takeovers. If a buyer crosses a set ownership threshold, other shareholders can buy extra shares at a discount, diluting the buyer and making the takeover far more expensive.
When an unwanted bidder starts buying up a company's shares, the board has tools to fight back. The best known is the poison pill: a plan that floods the market with cheap new shares for everyone except the raider. It rarely gets triggered, but its threat forces bidders to negotiate. This entry explains how poison pills work and what they mean for shareholders.
- Category
- Fundamentals and Macroeconomics
- Reading time
- 3 min
- Also called
- Shareholder rights plan, Rights plan, Takeover defence
- Updated
Poison pills in plain words
A shareholder rights plan, commonly called a poison pill, is a company's defence against a hostile or unsolicited takeover.1
It aims to deter takeovers by making the company less attractive, usually by diluting the acquirer's ownership.1
A hostile takeover is one the target's board and management don't approve.2
How a poison pill works, step by step
Board adopts the plan
Often when a raider appears.
A threshold is set
E.g. 10%–20% ownership.
Raider crosses it
The pill is triggered.
Others buy cheap shares
Usually at half price.
Raider is diluted
Its stake shrinks. See Share Dilution.
Bidder negotiates
With the board instead.
Worked examples
Example 1 — Dilution
Stake roughly halvedA raider owns 15% of 100m shares. A pill lets all other holders buy one new share each for every share held.
| New shares issued | 85m |
| Total shares | 185m |
| Raider's stake | 15m ÷ 185m ≈ 8.1% |
The pill dilutes the acquirer's ownership, making control far harder to gain.1
Example 2 — Cost to the raider
≈ $638m moreTo regain 15% after dilution, the raider must buy shares at $50.
| Shares needed for 15% of 185m | ≈ 27.75m |
| Extra to buy | ≈ 12.75m × $50 ≈ $638m |
Poison pills make a takeover much more expensive. See Mergers and Acquisitions.
Example 3 — Negotiated deal
Better priceFacing a pill, the bidder raises its offer from $45 to $52 and the board agrees.
| Premium increase | +$7 a share |
The pill pushes bidders to negotiate with the board rather than buy around it.1
Takeover defences
| Aspect | How it works | Effect |
|---|---|---|
| Poison pill | Discounted shares for others | Dilutes the raider |
| Crown jewels | Sell the best assets | Makes target less attractive |
| White knight | Friendly buyer steps in | Alternative deal. See White Knight |
| Staggered board | Directors elected in waves | Slows control changes |
When poison pills are used
Hostile bids
Blocking unwanted takeovers.
Activist investors
Limiting stake-building. See Mergers and Acquisitions.
Negotiating leverage
Forcing a higher offer.
Protecting tax assets
NOL pills limit ownership changes.
Advantages and disadvantages
Advantages
- Gives boards time to evaluate bids.
- Can lead to higher takeover prices.
- Prevents creeping control.
Disadvantages
- Can entrench management.
- May block value-creating deals.
- Some investors view them negatively.
Going deeper
Read as far as you need. Each level builds on the one before it.
Rarely triggered
Beginner
The threat is usually enough; bidders tend to negotiate rather than trigger heavy dilution.
Other defences
Intermediate
Companies can also use defences such as a crown-jewels sale of their most valuable assets.2
Governance debate
Advanced
Investors debate whether pills protect shareholders or entrench boards; many pills now have expiry dates or shareholder votes. See Fiduciary Duty.
Common mistakes
Assuming a pill ends all bids
It forces negotiation, not always rejection.
Ignoring entrenchment risk
Boards may block good deals.
Forgetting dilution effects
Triggered pills reshape ownership.
Confusing it with buybacks
Different tools.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- A poison pill defends against hostile takeovers.
- It lets other holders buy discounted shares.
- The raider is diluted and must pay more.
- It pushes bidders to negotiate.
- Critics say it can entrench management.
Frequently asked questions
Are poison pills legal?
Why is it called a poison pill?
Do poison pills hurt shareholders?
Further learning
- Share Dilution
How the pill works.
- Dual-Class Shares
Another control mechanism.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- Corporate Finance Institute. Poison Pill (opens in a new tab) Checked .
- Corporate Finance Institute. M&A Glossary (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.