CSL Ltd (ASX: CSL) shares had another shocking month in May.
The biotechnology giant's shares ended April at $124.37. But by the close of play in May, they had sunk to $97.12.
That means CSL shares lost approximately 22% of their value across the month. So, what happened?
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Guidance downgrade spooks investors
The main catalyst for the selloff was CSL's update on 11 May.
The company revealed the findings of interim CEO Gordon Naylor's 90-day review and announced a downgrade to its FY 2026 outlook.
CSL now expects revenue of around US$15.2 billion and NPATA of around US$3.1 billion, excluding restructuring costs and impairments, on a constant currency basis.
Management said its growth initiatives are working, but the financial benefits will take longer than previously expected to materialise. Naylor commented:
Our growth initiatives are working, but the financial benefits will take longer than previously anticipated to materialise. As a result, we have now revised down our 2026 financial year guidance. CSL's culture and people continue to be first class, the industry is stable and growing and the company has evident strengths in plasma collections and influenza vaccines. I am confident that the company can be returned to profitable growth and my work is to position the business and the next CEO for success.
That was not the message investors wanted to hear after a difficult period for the company.
There were several pressure points behind the downgrade. In U.S. immunoglobulin, demand is still growing at mid to high single digits, but revenue is being affected by the normalisation of channel inventory. CSL expects this to have an impact of approximately US$300 million.
Albumin in China is also weighing on the outlook. While CSL said its market share has expanded and volumes have stabilised, market value has declined, leading to an expected revenue impact of approximately US$200 million.
Other factors, including the Middle East conflict, revised HEMGENIX growth, and competition in iron, are expected to have a combined impact of approximately US$150 million.
Impairments add to the pressure
The downgrade was not the only issue weighing on the share price.
CSL also advised that it expects to recognise approximately US$5 billion of additional non-cash, pre-tax impairments across FY 2026 and FY 2027.
These impairments relate to CSL Vifor intangible assets, including the product portfolio, as well as under-utilised property, plant and equipment.
For investors, this was another reminder that some past investment assumptions have not played out as hoped. CSL also acknowledged that the Vifor acquisition has underperformed expectations and that invested capital has grown faster than earnings.
That has added to concerns about returns, capital allocation, and how long it may take for CSL to rebuild investor confidence.
Can CSL recover?
Despite the heavy selling, management has not given up on the long-term story.
The company said it is making progress on portfolio and commercial execution, operational simplification, and its transformation program.
It also highlighted strengths in plasma collection and influenza vaccines, as well as early signs of improvement in end-patient demand and momentum across recent product launches.
But the market's response in May was clear. Investors wanted stronger earnings momentum, not another reset.
CSL remains a global healthcare leader with valuable assets and a long history of innovation. But after such a brutal month, the company may need to show tangible progress before its shares can regain the market's trust.
