Reduce your heavy periods naturally with no side effects:

Exercise daily.

Eat well.  A few fruits a day and many, many veggies a day (5-7 servings).  Replace bread, milk and meat with leafy greens, beans, seeds, and nuts.  Cut out refined sugar and processed food, as well as fast food. 

Get your vitamin and mineral levels tested (blood test).  If deficient, choose reputable supplements to get those levels back up to normal.  Eat foods high in those vitamins and minerals! 

Optimal health = balanced hormones = lighter periods...period!

Here's some good information about vitex from about.com, has a lot of side effects   http://altmedicine.about.com/od/herbsupplementguide/a/Vitex.htm

It's not a vitamin, it's an herb, and most, if not all herbs can have dangerous side effects.  Personally, I stay away from herbal medicine and stick to vitamins and minerals.

Vitamins and minerals from foods are the safest bet.  There are no side effects from eating 5-10 servings of fruits and vegetables a day.  Vitamin and mineral supplements should be researched in depth from reputable websites and books, as well as getting info from a doctor or nutritionist before taking.  Too much in supplement form can be toxic.