Citadel, the multi-strategy hedge fund major founded by Ken Griffin, has taken a £305m ($384.5m) short position in GSK, which equates to 0.515 of the British pharmaceutical giant's stock, according to a report by the Financial Times.

The report cites Citadel as disclosing the short position – the largest ever in the drugmaker and the first since 2013 – in regulatory filings made with the UK Financial Conduct Authority (FCA).

Neither Citadel nor GSK have commented on the move, with the drugmaker maintaining its usual stance of not discussing individual shareholders.

The short bet comes just after GSK announced a £2bn share buyback alongside its fourth-quarter earnings, in which the company also raised its long-term sales target to nearly $50bn. Despite these measures, GSK’s stock declined 7.5% in 2024, underperforming rival AstraZeneca, which saw a 1.3% drop over the same period.