"Acetaminophen use on the majority of the days over many years appears to be associated with this new adverse effect," Walter said. "However, the study does not allow one to conclude a causal relationship," he added.
According to the researchers, animal studies have linked acetaminophen to toxic effects on bone marrow, which could be why there is a greater risk for blood cancers with long-term use of the drug.
Bonnie Jacobs, a spokeswoman for McNeil Consumer Healthcare, a division of McNeil-PPC, Inc., the maker of Tylenol, noted that "Tylenol has over 50 years of clinical history to support its safety and efficacy."
She added, "We appreciate the assessment of the authors that further studies would be needed before it is possible to draw any conclusions about the use of acetaminophen, and we welcome additional research in this area."
Walter noted that the risk of blood cancers is relatively small; an estimated combined total of 137,260 people in the United States were diagnosed with leukemia, lymphoma or myeloma in 2010. "New cases of leukemia, lymphoma and myeloma are expected to account for 9 percent of the new cancer cases diagnosed, so even a doubling of that risk is still relatively small," he said.
Deborah Banker, vice president of research communications at the Leukemia & Lymphoma Society, said that "it is unclear why chronic lymphocytic leukemia/small lymphocytic lymphoma should have different risk associations with acetaminophen use."
Eric Jacobs, strategic director of pharmacoepidemiology at the American Cancer Society, said that "there is no evidence from this study that less frequent or shorter-term use might raise risk of these cancers. Replication of this finding in additional high-quality studies is needed."
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