Drinking just two alcoholic drinks a day when you have breast cancer fuels the growth of tumours, a study says.
Alcohol has been linked to cancer before
It has long-been known alcohol increases the risk of developing cancer but the effect of drinking once cancer is present is less established.
A University of Mississippi team found giving mice the equivalent of two to four drinks a day doubled the normal growth of a tumour after four weeks.
Cancer patients are often just told to moderate drinking.
In the study, researchers gave female mice the human equivalent of two drinks a day for four weeks, while a control group were given no alcohol.
The mice were then injected with breast cancer cells.
Within four weeks, the tumours in the alcohol-fed mice weighed 1.4g on average, almost twice the size of tumours in the control group.
The team, which presented the research to the American Physiology Society, said alcohol caused cancer cells' blood vessels to grow which in turn fuelled the growth of the tumour.
The mouse study builds on an earlier research with chicks that showed alcohol consumption increased the expression of a protein known as VEGF.
VEGF fuels tumour growth by spurring the development of blood vessels in cancer cells that might otherwise die.
Normally, the immune system can kill off small tumours. However, when they grow large enough the body can no longer fight off the tumour cells.
Lead researcher Jian-Wei Gu said: "The vast majority of tumours result from over expressed VEGF.
"Every day, we produce a lot of cancer cells, but they don't become bigger.
"But if the cells establish blood vessels, the tumour grows and strengthens, a process known as angiogenesis."
And he added he would advise patients not to drink if they were undergoing cancer treatment.
"I don't think two to four drinks per day is okay.
"The public needs to know of these results."
Ed Yong, Cancer Research UK's science information officer, said: "The link between alcohol and breast cancer, and many other cancers for that matter, is well known.
"But this is the first time I have heard of the impact of alcohol once cancer is there."
He said more research was needed to see if the findings were replicated in adults and whether it also applied to other cancers.