How effective is Dianette or Diane-35?

I am assuming you are asking about the chance of pregnancy since this drug is not manufactured solely for the purpose of preventing a pregnancy it is hard to find anything that comes right out and says it prevents pregnancy 99% of the time. It is not sold in the US. Canada and GB don't advise it's use for birth control - although I read there was a huge campaign in early 2000 that introduced it for that purpose. I've added a link to a page with tons of information and the following information available from GB and the HC sites.

The Health Canada advisory says Diane-35 (and Dianette):
Dianette is not indicated for use solely as an oral contraceptive.
Dianette needs to be stopped 3 to 4 cycles after the treated condition has completely resolved.

Dianette is useful for the treatment of Poly-cystic Ovary Syndrome in women. (PCOS)
The Health Canada advisory says Diane-35 should not be used solely as an oral contraceptive.

Dianette is contraindicated in women with a personal or close family history of confirmed, idiopathic *VTE. The advisory says those who use Diane-35 have an increased risk of developing *VTE, which includes pulmonary embolism and deep vein thrombosis. The incidence of *VTE in Dianette users is higher than that in women who use low-dose Estrogen BCPs or oral contraceptives.

*Blood Clots