Quick facts
- Formal name: shareholder rights plan
- Upheld in Delaware: Moran v. Household International (1985)
- Statutory basis: 8 Del. C. §§ 141, 151, 157
- Typical trigger: 15% to 20% ownership
- Adopted by: the board, without a shareholder vote
A poison pill, formally a shareholder rights plan, is a defence that makes a hostile takeover prohibitively expensive. The board issues rights to all shareholders in advance. If a buyer crosses a set ownership threshold without board approval, those rights become exercisable for everyone except the buyer, at a steep discount. The buyer's stake is diluted and the acquisition stops making financial sense.
Both names describe the same instrument. "Shareholder rights plan" is what appears in the filing. "Poison pill" is what everyone calls it.
In plain English
A pill does not prevent a takeover. It redirects one. Without a pill, a buyer can go straight to shareholders and buy their shares. With a pill, the buyer has to come to the board first, because the board is the only party that can switch the pill off.
It converts a purchase into a negotiation.
How it works, step by step
- The board adopts the plan and issues rights to every shareholder.
- The rights attach to the shares and trade with them. Nothing happens yet.
- A trigger threshold is set, commonly 15% to 20% of outstanding stock.
- A buyer crosses the threshold without board approval.
- The rights become exercisable for all shareholders except the buyer, allowing them to buy stock well below market value.
- The share count rises, the buyer's percentage falls, and the cost of the takeover climbs sharply.
Twitter's plan is a clean worked example. On 15 April 2022, after an unsolicited proposal to acquire the company, the board unanimously adopted a limited duration rights plan. The rights become exercisable if any person or group acquires beneficial ownership of 15% or more of outstanding common stock in a transaction the board has not approved. Once triggered, each right lets its holder buy additional common stock with a market value of twice the exercise price, and the acquirer's own rights become void. The plan was set to expire on 14 April 2023.1
That is the whole mechanism, in a company's own words, filed with the Securities and Exchange Commission.
Flip-in and flip-over
Flip-in is the common feature described above. Shareholders buy the target's own shares cheaply, diluting the acquirer inside the target.
Flip-over applies if the target is later merged into the acquirer. It lets target shareholders buy the acquirer's shares at a discount, which discourages completing a takeover through a back-end merger.
Variants you will see
Standard pill. Trigger of 15% to 20%, duration of one to three years.
Low-trigger pill. A threshold nearer 5% to 10%, used when management fears opportunistic accumulation during a price collapse. These attract more scrutiny, because the lower the trigger, the more it looks like protection of incumbents rather than of shareholders.
Net operating loss pill. A deliberately low trigger designed to preserve tax assets, which can be impaired if ownership shifts too far. The justification here is tax law rather than takeover defence, and it is treated differently for that reason.
Shelf pill. Drafted, approved in principle, and left unadopted so it can be deployed within days. Many large companies keep the paperwork ready rather than carrying an active plan.
Why it is lawful
Moran v. Household International is the case. Household's board adopted a rights plan triggered when a person became the beneficial owner of 20% or more of its stock. A shareholder sued, arguing the board had no authority to do it and that the plan stripped shareholders of the right to receive tender offers and to run proxy contests.
The Delaware Supreme Court disagreed on every point. It held that the directors adopted the plan "pursuant to statutory authority in 8 Del. C. §§ 141, 151, 157" and that they received the benefit of the business judgment rule in doing so.2 Those sections are the board's general management authority, the power to set the terms of a series of stock, and the power to create rights and options to acquire shares.34
The court also made a point that gets quoted less often and matters more. Approving the adoption of a pill is not approval of how it is later used:
The ultimate response to an actual takeover bid must be judged by the Directors' actions at that time, and nothing we say here relieves them of their basic fundamental duties to the corporation and its stockholders.2
Adopting a pill is protected. Hiding behind one indefinitely is a separate question that gets its own review.
The court further reasoned that pre-planning a defence deserves more deference rather than less, because a board that prepares in advance is less likely to make a panicked decision under the pressure of a live bid.2
What it costs the board
A pill is not free. Proxy advisers and large institutional shareholders scrutinise them, particularly long-duration plans and low triggers, and boards are frequently asked to justify the specific threat and to keep the duration short. Adopting one signals that management expects a fight, and it invites the argument that the board is entrenching itself rather than protecting value.
What an acquirer does about it
A pill removes the direct route, not every route. A determined buyer can:
- negotiate with the board and ask for the pill to be redeemed
- run a proxy fight to replace directors with ones who will redeem it
- wait for the plan to expire
- raise the price until refusing becomes difficult to defend
This is why a staggered board makes a pill much more powerful. If only a third of directors can be replaced each year, the proxy route takes two annual meetings instead of one, and the pill holds through both.
What founders should take from this
If you run a private company, you do not have a pill and do not need one. What transfers is the structural lesson: defences work when they are already documented before the threat arrives. The Delaware court's own reasoning was that pre-planning deserves protection precisely because decisions made under bid pressure are worse.
The corollary is that the control provisions in your charter and financing documents are the version of this you actually have, and they were negotiated years before anyone tried to use them.
What to do next
Read the hostile takeover guide for how a pill sits alongside the other defences and what the first days of an unsolicited bid look like.
Related reading
- White Knight covers the friendly alternative a pill buys time to find
- Proxy Fight covers the route around a pill
- Tender Offer covers the direct approach a pill blocks
- Staggered Board covers the defence that makes a pill hold longer