NEW YORK (CNNMoney.com) - New data from drug maker Merck suggests that heart risks associated with painkiller Vioxx begin much earlier than it had previously suggested, said a report published Thursday.

The data suggest that cardiovascular risks increase for Vioxx users after four months of use, rather than 18 months as the company had previously said, according to the Wall Street Journal.

Merck initially withdrew Vioxx from the market in September 2004, citing data from a study called Approve that showed an uptick in incidences of "confirmed thrombotic cardiovascular events" after 18 months, said the report.

But a new summary of follow-up data finds that Vioxx patients had an increased risk after only four months, according to the Journal.

In a graph from the earlier study, the lines tracking events among Vioxx patients and placebo takers diverged after 18 months, rather than four, said the Journal.

In a statement, Merck said that the data "do not establish that the risk for Vioxx starts earlier than had previously been reported," said the report.

Around 11,500 lawsuits have been filed against Merck by Vioxx patients alleging heart complications from taking Vioxx, said the Journal.

Shares of Merck (Research) fell slightly to $34.30 in electronic trade before Thursday's market open.

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