Allergan, the US maker of Botox, has rejected a takeover bid from Valeant Pharmaceuticals, saying it "substantially undervalues" the company and questioning the Canadian firm's long-term growth plan.

Allergan's board of directors unanimously turned down the unsolicited offer from Valeant, worth nearly $46 billion, highlighting its significant risks.

"After careful review and consideration, our board of directors has unanimously determined that Valeant's unsolicited proposal substantially undervalues Allergan," said David Pyott, Allergan's chairman and chief executive, in a statement.

The proposal "does not reflect the value of the company's leading market positions, sales and marketing foundation, industry-leading research and development efforts, as well as future revenue and earnings growth," he added.

In addition, the Irvine, California-based company, best-known for its Botox wrinkle treatment, said it expected earnings per share growth of 20-25% and continued double-digit sales growth.

The Allergan board determined that Valeant's proposal "creates significant risks and uncertainties for Allergan's stockholders and believes that the Valeant business model is not sustainable."

On 22 April, Valeant Pharmaceuticals, backed by activist shareholder William Ackman, proposed a stock-and-cash deal buyout valuing Allergan at $45.6 billion.

Valeant said the acquisition would enable huge cost savings and create a leader in opthalmology, dermatology, aesthetics and other growing health fields.

Under the terms of the proposal, Valeant would buy each Allergan share for $48.30 in cash plus 0.83 shares of Valeant. At the time, Valeant's CEO, J. Michael Pearson, said the proposal represented a "substantial" premium to Allergan's valuation.

In rejecting the bid, Allergan, highlighted its large stock component as a risk for Allergan stockholders "due to the uncertainty surrounding Valeant's long-term growth prospects and business model."

"In particular, we question how Valeant would achieve the level of cost cuts it is proposing without harming the long-term viability and growth trajectory of our business. For those reasons and others, we do not believe that the Valeant business model is sustainable."

Valeant, headquartered in Laval, Quebec, was not immediately available to comment on Allergan's rejection.