Most side effects of the pill are quite common and not serious, but some of them are very serious although not as common.

  1. Spotting.  Because the pill causes the lining of the uterus to become thinner and less hospitable for a fertilized egg to implant, bleeding may occur between periods.  This is most notable in the first three months of using the pill.
  2. Tender Breasts. Many women experience tender breasts and enlargement of breasts due to the pill.  If during this time, you find a lump in the breast, you should see a doctor. 
  3. Weight Gain. Some women retain more fluids when on the pill and will gain weight.
  4. Nausea. Because the pill is a hormone, some women experience nausea, especially if they take the pill on an empty stomach.
  5. Headache.  Common side effects are headaches and sometimes migraines.  See a doctor if these are severe.
  6. Changes in Mood. A study1 of nearly 100 women showed changes in the brain as a result of the pill, although further study is needed to determine if this causes mood changes.  Some women have reported feelings of depression.
  7. Less Interest in Sex. Some women show less interest in sex when on the pill.  Many factors influence libido however.
  8. Heart Attack or Stroke.   The pill can cause a heart attackor stroke as a result of a blood clot. Most women under the age of 35, who are not obese, and do not smoke, are at low risk however.  Women with any type of heart disease should not take birth control pills.
  9. Cancer.  There have been over 50 studies on this topic and a number of them have found that the pill does cause an increase in the chance of breast cancer.  If a woman has a family history of the disease, she should not take the pill.

1 Human Brain Mapping