Antibiotics Linked to Breast Cancer

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Antibiotics Linked to Breast Cancer

The research, led by researchers at Group Health Cooperative in Seattle, is published in the Journal of the American Medical Association.

Scientists followed the progress of more than 10,000 women for an average of 17 years.

Their research suggests that women who use a lot of antibiotics have an increased risk of breast cancer.

Those who took antibiotics for more than 500 days during the study had twice the risk of those who took none.

The risk was smaller for women who took fewer antibiotics.

But even women who had between one and 25 prescriptions over an average period of 17 years had an increased risk. They were about one and a half times more likely to get breast cancer than those who took no antibiotics were.

Lead researcher Dr Christine Velicer said more work was required to evaluate whether the increased risk is down to the antibiotics themselves.

She said: “It may be that women who take a lot of antibiotics have some other processes happening in their bodies – such as a weak immune response or a hormonal imbalance – which may be an underlying cause of breast cancer.”

Dr Velicer said women who never took antibiotics may be generally healthy, and so less likely to develop cancer, as well as other diseases.

“This possibly could account for some – but probably not all – of the increased breast cancer risk we saw among antibiotic users.”

Dr John Potter, of the Fred Hutchinson Cancer Research Center, also worked on the study.
He said it was far too soon to blame antibiotics for an increased risk of cancer – and stressed that they still had an important role to play in the treatment of infection.

However, he also emphasized the fact that the study had uncovered an association between breast cancer and a broad range of antibiotics.


There are various theories about how antibiotics may increase the risk of breast cancer.
One is that antibiotics affect the bacteria in the intestine, interfering with the metabolism of certain foods that are known protect the body against cancer.

Others theories focus on antibiotics’ impact on the body’s immune and inflammatory responses.

Dr Michelle Barclay, of the charity Breakthrough Breast Cancer, said: “Breast cancer is an enormously complex disease and we don’t yet know all the causes. There may be many factors that increase a woman’s breast cancer risk and this study provides food for thought. However, caution should be exercised when interpreting results, as the reason for taking antibiotics may be the actual risk factor, rather than the drug itself.”

Dr Michelle Barclay said, “Women should not be deterred from taking antibiotics if their GP thinks it is necessary.”

Breast cancer is the most common cancer in women worldwide.

The above was last updated 7/17/2009

Feedback on Antibiotics

Subj: Zithromax and sex
Date: 5/24/2004
I was wondering if I finished up my antibiotics (Zithromax) last night, and I’m on the Pill…is it safe for me to have sex tonight?


AskDocWeb: Yes it is safe.

Subj: Abdominal pain
Date: 11/16/2005
I used to take the antibiotic erythromycin without any side effects but when I take them now I get a strong abdominal pain. Why is this occurring now and not before? Thanks,


AskDocWeb: You can have a reaction to prescription drugs at any time, it just happens. Report this to both your doctor and pharmacist so no one will prescribe this med to you again.

Subj: Bronchial cough in child
Date: 3/10/2006
My 3 year-old son weighs about 35 pounds and has a persistent bronchial cough. I purchased pediatric amoxicillin (250 mg / 5ml) powder. How much should I add to how much liquid? And how often should I give it to him?


AskDocWeb: Please ask your physician. This website is for feedback about health products, not for medical advice.

Subj: False positive for cocaine
Date: 3/30/2006
I have been told that amoxicillin can cause a false positive for cocaine on a drug test. Can you give me any information on this and where I can find an expert on this subject. Thank you for your time.


AskDocWeb: We’re not sure where you might find an expert on this but we’ve discovered that many medications false positives on drug tests. You can read our notes here.

Subj: Amoxil and breastfeeding
Date: 5/20/2007
I need to take 2grms of Amoxil before dental treatment. How long should I wait before resuming breastfeeding please?


AskDocWeb: Even though Amoxicillin is excreted into breast milk it is still considered as “usually compatible with breastfeeding” by the American Academy of Pediatrics.

One problem with most antibiotics is that they can produce excessively loose motions or diarrhea in the baby. Some infants appear more unsettled with tummy aches or colic but these effects are not clinically significant and do not require treatment. Typically the value of breastfeeding outweighs the temporary inconvenience. One theory is that exposure may sensitize the baby to later doses but this is exceedingly rare.

Research out of the University of Iowa suggests a link between the use of amoxicillin when fluoride is present in the water and dental problems in permanent teeth of the nursing infants. There seems to be a significantly elevated risk for developmental enamel defects, barely noticeable white flecks, pits and brown stains. The duration of amoxicillin exposure use was related to the number of early-erupting permanent teeth in infants.

The findings suggest that amoxicillin use in infancy could carry some heretofore undocumented risk to the developing teeth. While the results of the study do not warrant recommendations to cease use of amoxicillin early in life, they do show the need to use antibiotics judiciously during infancy.

Note: Research out of the University of Iowa was sponsored by grants from the National Institutes of Health, Bethesda, Md. Results of the study were published in the October issue of Archives of Pediatrics and Adolescent Medicine. The AAP considers amoxicillin to be “compatible” with breast-feeding.

Subj: Severe yeast infection
Date: 6/1/2007
I was recently prescribed Biaxin for Strep Throat and took 1000 milligrams for 10 days. I had mild upset stomach throughout the duration of the prescribed treatment. I am also taking Yasmin for hormone irregularity. I have yet to have my period and came down with a severe yeast infection. Is this normal?


AskDocWeb: Yes, this is something that is normal and it should go away. Do use Monistat or something like it for the yeast infection.

Subj: Amoxicillin false positive cocaine?
Date: 11/15/2007
How long does Amoxicillin stay in the body and cause a false positive cocaine test? I have a friend who just tested positive for cocaine (350 ng/ml), and he had taken Amoxicillin for two weeks up until a week before the urine screen. Could this have caused a false positive with a number that big?


AskDocWeb: That is unlikely. Amoxicillin has a short half-live and 95% of it is metabolized within 12 hours. Traces may remain for a couple of days but not two weeks.

Could i be pregnant?
I have been taking Ortho Tri-Cyclen for almost 9 months. On November 25th i had gotten a penicillin shot for strep throat. Then on December 1st i had unprotected sex for five minutes then switched to a condom. Could i be pregnant?


AskDocWeb: There is a small chance of it but yes, it is possible. Penicillin may reduce the amount of hormones absorbed from birth control pills, making them less effective, which is why they recommend using a backup form of birth control when taking antibiotics.