it wont lower the effectiveness of preventing pregnancy, but she will become a moody, depressed, crying everyday, raving maniac if she doesnt have PMDD. I took Yaz for over a year and am off of it now for around a month and I AM STILL recovering from the emotional side effects this horrible drug gave me. I used to be a normal, sweet girl. Now I cry every day, get upset about nothing, and am just a miserable person. ALSO, Yaz has very dangerous levels of potassium in it which have led to VERY young women to suffer from heart attacks and strokes. STAY AWAY FROM YAZ!