Drugmaker AstraZeneca has seen its share price plummet more than 16% after announcing disappointing results.
The firm said first-quarter revenue fell 10% to $5.05bn (£3.84bn).
It also reported a major setback in trials of a new lung cancer drug therapy.
The company was by far the biggest loser on the benchmark FTSE 100 share index, which was up 3.63 points, or 0.05%, to 7,455.95 in Thursday morning trading.
Top gainer on the index was drinks giant Diageo, which rose 7.5% after reporting higher full-year sales and profits.
The maker of Johnnie Walker whisky and Smirnoff vodka reported sales of £12.05bn for the year ending 30 June, a rise of 4% on an organic basis, while operating profit rose to £3.6bn.
Mining firm Anglo American also made strong gains, adding 3.4% after saying it was resuming dividend payments six months early on the back of healthy interim results.
In the currency market, the pound edged higher against the dollar, up 0.18% at $1.3145. Against the euro, the pound was 0.51% higher at 1.1229 euros.