Botox maker Allergan has its work cut out for it.

The company, which is looking to fend off a hostile bid by Valeant and billionaire Bill Ackman, must clear this bar: it has to prove the principal reason for its offer to buy smaller rival Salix Pharmaceuticals isn’t to thwart the acquisition, according to a merger law expert.

Allergan is reportedly nearing an all-cash deal to buy Salix, a move that would make it more difficult for the bid by Ackman and Canada-based Valeant to win the day.

“Allergan cannot buy another company to make themselves a less attractive takeover target for the sole purpose of thwarting this bid,” said Jonathan Macey, a law professor at Yale University.

On Tuesday Ackman — the company’s biggest shareholder with a 9.7 percent stake — said he will sue Allergan if it attempts to acquire Salix without a shareholders vote.

“By undertaking an acquisition without a shareholder vote, with the purpose and desired effect of frustrating a Valeant transaction, you are breaching your commitment that shareholders would have a vote on the value proposition offered by Valeant,” he wrote in a letter to the Allergan board.