A leading stent manufacturer has admitted that there is an increased clot risk for those with Boston Scientific’s cardiac stent. It is the first acknowledgment by a stent maker of an increased risk of the dangerous side effect of stents, which are small wire-mesh tubes used to keep previously clogged arteries open. Drug-coated stents, which are coated with medicine to prevent the growth of tissue after implantation, have sales exceeding $5 billion a year in a market dominated by Boston Scientific and Johnson & Johnson.