Shares in the pharmaceuticals giant sank 3.79 percent to 2.694 pence in early afternoon trading, while London's FTSE 100 index of top shares slipped 0.85 percent to 5,650.60 points.
The company, the third-biggest European drugmaker, said in a statement on Wednesday that it was "disappointed" by the judgement delivered by a Missouri court and would appeal.
The news prompted a raft of earnings downgrades from analysts, who were expecting the product to remain exclusive until September next year, when the patents were due to expire.
The decision means cheaper, copycat versions of the drug are likely to arrive 18 months earlier than expected, affecting sales and earnings in the next few months.
Toprol XL, a treatment for angina and high blood pressure, generated sales of 1.3 billion dollars in the United States last year. It accounts for around 5 percent of the group's total sales.
But the generic drug makers it was fighting - KV Pharmaceutical Co, Andrx Corp and Eon Labs, part of Novartis AG - were likely to hurry to get their cheaper versions to the market, analysts said.
"We anticipate near term approval of one or more of the generic challengers leading to collapse in both Astra Zeneca's US market and pricing," broker Morgan Stanley told clients this morning.