毒丸防御（英语：poison pill），又称“股权摊薄反收购措施”或“股东权益计划”，是目标公司抵御恶意收购的一种防御措施，1982年由美国Wachtell, Lipton, Rosen & Katz律师行的并购律师马丁·利普顿（Martin Lipton）发明，在美国是经过1985年特拉华州大法官法庭(Delaware Chancery Court)的判决才被合法化的，由于它不需要股东的直接批准就可以实施，故在八十年代后期被广泛采用。

实施毒丸防御的公司，由公司董事会事先通过一项股权摊薄条款，一旦敌意方收购公司一定比例的股份（通常是10%至20%的股份），即触发该条款生效，使公司原有股东可以较低的价格获得公司大量股份，从而抬高收购方的成本。

A shareholder rights plan, colloquially known as a "poison pill", is a type of defensive tactic used by a corporation's board of directors against a takeover. Typically, such a plan gives shareholders the right to buy more shares at a discount if one shareholder buys a certain percentage or more of the company's shares. The plan could be triggered, for instance, if any one shareholder buys 20% of the company's shares, at which point every shareholder (except the one who possesses 20%) will have the right to buy a new issue of shares at a discount. If every other shareholder is able to buy more shares at a discount, such purchases would dilute the bidder's interest, and the cost of the bid would rise substantially. Knowing that such a plan could be activated, the bidder could be disinclined to take over the corporation without the board's approval, and would first negotiate with the board in order to revoke the plan.