In England and Wales and other Commonwealth or common-law countries American Cyanamid Co. v. Ethicon Ltd.,  A.C. 396 (appeal taken from Eng.), is often viewed as the leading decision laying out the framework for deciding whether to grant a preliminary injunction. As I note in my book (p.176):
In the principal opinion, Lord Diplock concluded that the moving party is not required to prove a probability of success or prima facie case; rather, he must convince the trial judge only that “the claim is not frivolous or vexatious, in other words, that there is a serious question to be tried.” If the moving party succeeds on this front, the court then weighs the balance of conveniences, considering both whether the moving party “would be adequately compensated by an award of damages for the the [sic] loss he would have sustained as a result of the defendant’s continuing to do what was sought to be en joined [sic] between the time of the application and the time of the trial.” If not, then the court should consider whether the defendant “would be adequately compensated under the plaintiff ’s undertaking as to damages for the loss he would have sustained by being prevented from doing so between the time of the application and the time of the trial . . . . [W]here there is doubt as to the adequacy of the respective remedies in damages available to either party or to both . . . the question of balance of convenience arises.” I also noted that the decision has come in for criticism from, among others, Sir Robin Jacob, who argues that Cyanamid notwithstanding courts often do take likelihood of success into account.
Relevant to this debate, Gang Mei has recently published a paper titled Interlocutory Injunctions in IP Infringement Actions in England and Wales and in Ireland – American Cyanamid Revisited in 46 IIC 175-202
The American Cyanamid decision recognizes that the object of an interlocutory injunction is the avoidance of irreparable injury to rights. This article revisits the decision in the context of IP infringement actions. It finds that the Cyanamid guidelines have been both misunderstood and misapplied. It also finds that the guidelines themselves are inadequate to achieve the identified object because, while rightly making the extent of each party’s potential irreparable injury a primary factor in the balance of convenience, they sideline another equally relevant factor, namely, the probability that such an injury will occur.
Dr. Mei argues, among other things, that courts should take into account such considerations as "the strength of the plaintiff's case" (which he contends "should become a primary factor in the balance of convenience," consistent with Lord Diplock's approach in another case, NWL v. Woods), whether the preliminary injunction hearing will be decisive, and whether a permanent injunction would be available at the conclusion of trial. He also argues that courts should not place much if any weight on preserving the status quo, a matter that (quoting John Leubsdorf) he characterizes as "a habit without a reason."