The CSL Ltd (ASX: CSL) share price underperformed the S&P/ASX 200 Index (ASX: XJO) in 2024.

While the benchmark index gained around 7.5% in the year just past, shares in the ASX 200 biotech stock closed down approximately 1.2%, trading for $286.65 a share on 31 January. Though that doesn't include the $3.97 a share in unfranked dividends eligible investors will have received over the year.

Here's why the CSL share price looks to have caught some headwinds.

Why did the CSL share price lag the ASX 200 in 2024?

Heading into August, the ASX 200 biotech stock was well up for the calendar year.

On 12 August, the CSL share price closed the day in the green at $308.93.

But things took a turn for the worse the following day, when the company released its FY 2024 results, the last price sensitive news we've had to date.

Amid high expectations for those results, the CSL share price closed down 4.6% on the day.

Investors were favouring their sell buttons, and have continued to pressure the stock since, despite the company reporting an 11% year on year increase in revenue (in constant currency) to US$14.8 billion.

And profits were well up, with net profit after tax before amortisation (NPATA) in constant currency up 15% to US$3.01 billion.

This saw management boost the full year dividend payout by 12% from FY 2023 levels.

But with some brokers having forecast moderately stronger financial metrics, even the company's growth outlook for FY 2025 failed to boost the CSL share price, which is now down some 8% since 12 August.

As for that FY 2025 outlook, management forecast year on year revenue growth in the range of 5% to 7% in constant currency. And NPATA is expected to come in between US$3.2 billion to US$3.3 billion in constant currency. That's 10% to 13% above FY 2024 levels.

Commenting on the company's outlook at the time, CSL CEO Paul McKenzie said:

The momentum in our CSL Behring business is expected to continue to be underpinned by the strong patient demand in our immunoglobulins franchise…

Over the medium term, CSL is in a strong position to continue to deliver annualised double-digit earnings growth.

McKenzie also highlighted that the company has "a number of initiatives underway in plasma collections and our manufacturing operations that will continue to drive efficiencies and lead to an improving CSL Behring gross margin".

The CSL share price is flat in early afternoon trade today, with shares trading for $283.43 apiece.