Extended use of antibiotics in middle and early adulthood Your risk of developing precancerous tumors called polyps in the colon may rise, a large study suggests. 
Antibiotics can cause diarrhea in many people. This happens because of the antibiotics. This kills some of your normal gut bacteriaThis allows for an increase in the number of bacteria that is not needed.
Researchers in the U.S. followed the health of more than 120,000 nurses aged between 30 and 55, who were part of the Nurses Health Study which was launched in 1976. Researchers discovered that 1,194 nurses had abnormal growths in their colons and rectums between 2004-2010.
Participants who took antibiotics for more than two months between the ages 20 and 39 were not eligible. Polyps are 36% more commonAn adenoma is a type of cancer that has been found in patients who have not received a long course of antibiotics in their 20s or 30s.
Women who took antibiotics for more than two months in their 40s or 50s were not eligible. Polyps are 69% more common.
Similar results were seen in women who had taken antibiotics for at least 15 days between the ages of 20 and 39 and between the ages of 40 and 59. 73% More Most likely to be diagnosed as an adenoma.
Associate Professor Graham Newstead is the head of colorectal at the Prince of Wales Private Hospital and director of Bowel Cancer Australia.
It does appear that people who take too many antibiotics may be more at risk. [sic] People with more polyps are more likely to get them than those who have fewer.
Remember that not all polyps become cancerous. It is the polyps that cause the cancer. You may be slightly more likely to develop cancer if you have more polyps, or a tendency to get more polyps. 
According to the report, these were the words of the researchers:
Antibiotics have a profound effect on the microbiome of the gut. They reduce the number and diversity of bacteria and decrease resistance to harmful bugs.
These factors could all play a critical role in the development bowel cancer. In addition, bugs that require antibiotics can cause inflammation. This is a risk factor for developing bowel cancer.
These findings, if confirmed by additional studies, indicate the need to limit antibiotic use and inflammation that could drive tumor formation. 
Additional Risk Factors for Colorectal Cancer
Associate professor of medicine at Harvard Medical School Dr. Andrew Chan said that the study does not prove that antibiotics cause polyps. Only that there is a relationship. 
Chan stated that while Chan’s study was limited to women, it is likely that the same holds for men.
“More research is needed to understand the interaction between changes in one’s gut bacteria, and future colorectal cancer risk.” 
Other than polyps, there are many other risk factors that can lead to bowel cancer.
- Being overweight or obese
- Insufficient physical activity
- Red meat and processed meats are high in diets
- Grilling, boiling, and frying meats at high temperatures
- Heavy alcohol use
- Age 50 and older
- A personal history of colorectal carcinoma
- An individual’s history of inflammatory bowel diseases
- Polyps or colorectal cancer in the family
- Certain inherited syndromes
- Either African-American or Eastern European Jew
- Type 2 Diabetes 
Research suggests that night shift work may increase your chance of developing colorectal cancer. This could be due to fluctuations in melatonin levels.
 The Guardian
 BBC News