Adds closing share price moves for CSL in paragraph 2

By Rajasik Mukherjee and Roushni Nair

May 11 (Reuters) - CSL Ltd CSL.AX cut its annual profit forecast and warned of a $5 billion impairment on Monday, reigniting doubts about the merits of its Vifor acquisition and the pace of its turnaround, sending the biotechnology firm's shares to near decade lows.

CSL said it expects to recognise about $5 billion in non-cash impairments through 2026 and 2027 related to Vifor Pharma, a Swiss drugmaker it bought in 2022 for $12 billion.

The deal has been hit by regulatory hurdles and weak performance. CSL had already flagged $1.1 billion in after-tax impairments in February, partly linked to cuts in the value of intellectual property in its iron-deficiency and kidney medicines unit.

CSL, the former Commonwealth Serum Laboratories, has faced a succession of setbacks over the past nine months.

In August, it unveiled a sweeping overhaul including plans to cut up to 3,000 jobs, followed by a guidance downgrade two months later and a delay to the planned spin-off of its Seqirus vaccines unit.

The turbulence has continued in 2026 with the surprise retirement of Chief Executive Paul McKenzie in February, while the U.S. Pentagon's decision last month to drop a flu-shot mandate for troops dealt another blow to demand expectations and investor confidence.

CSL, once Australia's priciest stock, closed down 16% at its lowest since January 2017, in its worst session since August last year.

The shares are down more than 40% so far this year, sharply underperforming Australia's top 20 stocks index .ATLI, up nearly 5%, and wiping more than A$9 billion ($6.5 billion) off its market value, according to LSEG data.

CSL said it expects 2026 net profit, excluding restructuring and impairment costs, of about $3.1 billion, down from $3.3 billion last year and below its earlier forecast range of $3.45 billion to $3.55 billion.

"Back-to-back downgrades do raise legitimate questions about the company's visibility into its own business," said Mark Gardner, founder and CEO of MPC Markets, a Sydney-based investment advisory firm.

The company also cut its 2026 revenue forecast to $15.2 billion on a constant-currency basis, compared with $15.6 billion a year ago.

"The broader picture is a firm in the middle of a complex transformation - still without a permanent CEO, dealing with the fallout of an overpriced acquisition, and navigating real revenue headwinds in its two biggest markets," Gardner said.

($1 = 1.3814 Australian dollars)