Activist investor William Ackman teamed up with Canadian drugmaker Valeant Pharmaceuticals International to make a joint run at buying wrinkle-treatment maker Allergan.

This is according to security filings last night.

The move to acquire Allergan, which is worth around $42 billion, is highly unusual as activist investors typically buy stakes and then agitate for strategic change, such as a sale of the company.

One source familiar with their plans said it comes after Valeant tried for over a year to court Allergan but was turned away.

Ackman and his $13 billion hedge fund, Pershing Square, have a long record in pushing companies to do his bidding, which could help Valeant buy Allergan.

Allergan and a spokeswoman for Pershing Square declined to comment.

Valeant, which has been on a buying spree and most recently acquired Bausch & Lomb Holdings, said that a merger with Allergan would create an "unrivaled platform for growth and value creation," and that it would soon finalise its proposal and then announce it.

A deal would bring together two mid-sized pharmaceutical companies with expertise in skin care and eye care products.

Allergan, which also has a lucrative portfolio of ophthalmic drugs to treat conditions such as glaucoma and dry eye, is larger by revenue, reporting $6.3 billion in sales last year. Valeant reported $5.8 billion in revenue last year.

Allergan's longtime CEO, David Pyott, is also an avid dealmaker who built Botox into a blockbuster by continuously testing it for new uses.