Merck has claimed that increased risks of heart attack and stroke with the use of Vioxx only occur if the drug has been taken for at least 18 months.? A new study casts doubt on this claim.? The study known as Victor was conducted by researchers at Oxford University in England.? The study revealed that cardiovascular events took place within 12 months of taking the drug.? The study has been accepted by the New England Journal of Medicine for publication.