hi, I'm just wondering if the medicine phentermine can cause any side effects such as absence of period. I have had my tubes tied for 3 years, and my fiance and i are sexually active but use no protection., I have never missed or been late as far as my period. I felt like a crazy woman, when I went to the grocery store to buy a test.I have been taking phentermine for 3 weeks now, and am a week late. Can there be a correlation?