The S&P/ASX 200 Index (ASX: XJO) finished up a welcome 2.6% in August, with no thanks to these three plunging ASX 200 stocks.
One is a diagnostic and therapeutic product developer.
One is a building materials company.
And the third is a biotechnology company.
Now, to be clear, just because these three ASX 200 stocks had a horror month in August, doesn't mean they may not have some strong turnaround potential following the past month's selling action.
But we'll save that analysis for a different day.
As for the big losses suffered in August…
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ASX 200 stocks crashing in August
Starting with the third-worst ASX 200 stock performer of the month, we have Aussie biotech giant CSL Ltd (ASX: CSL).
CSL shares closed July trading for $270.90. On Friday, the last trading day of August, shares ended the day changing hands for $212.89, down 21.5%.
Most of that carnage came on 19 August, with shares closing down 16.9% after CSL reported its full-year FY 2025 results.
Belying the sell-off, most of the company's financial metrics were actually quite solid.
The ASX 200 stock reported FY 2025 revenue of US$15.6 billion, up 5% from FY 2024.
Earnings before interest, tax, depreciation and amortisation (EBITDA) were up 11% to US$5.3 billion.
And on the bottom line, CSL achieved 14% year-on-year growth in underlying net profit after tax and amortisation (NPATA) to US$3.3 billion.
But investors look to have gotten a bad case of the jitters after CSL announced its intention to spin off one of its three distinct business divisions. Namely, its Seqirus segment, which counts as one of the world's largest influenza vaccine businesses.
Management said they aim to demerge Seqirus into a separate and "substantial ASX-listed entity" before the end of FY 2026.
Moving on to the second ASX 200 stock best avoided in August, building materials company James Hardie Industries PLC (ASX: JHX).
James Hardie shares ended July at $41.31 and finished August trading for $31.17 apiece. This put the James Hardie share price down 24.6% over the month.
Pretty much all of the carnage occurred on 20 August, with James Hardie shares crashing 27.8% on the day after the company reported its first-quarter FY 2026 results.
Investors were going for their sell buttons after the company reported a 9% year-on-year decline in net sales for the quarter to US$899.9 million. Operating income of US$138.6 million was down a sharp 41%.
And James Hardie reported adjusted EBITDA of US$225.5 million, down 21% year on year.
Which brings us to…
Leading the way down in August
The worst ASX 200 stock performer on my list for August is diagnostic and therapeutic product developer Telix Pharmaceuticals Ltd (ASX: TLX).
Telix shares closed out July at $21.05 and finished August trading for $14.60 each, down a painful 30.6% over the month.
More than half of those losses were delivered on 28 August.
The Telix share price plunged 18.8% on the day after the company revealed it had run into a snag with the United States Food and Drug Administration (FDA).
The FDA said it had identified deficiencies relating to the Chemistry, Manufacturing, and Controls (CMC) package. The FDA is seeking more data before moving forward with Telix's Biologics License Application (BLA) for the company's TLX250-CDx (Zircaix) product.
Telix believes it will be able to address the remaining FDA requests "within a reasonable time frame".
