I wasn't familar with the name Kelnor so I looked it up and found that the other names it goes by.
Zovia 1/35e, Demulen 1/35-28, Kelnor 1/35-28, Kelnor 1/35, Zovia 1/50e, Demulen 1/50-28, Demulen 1/50-21
I haven't taken any of them so I really can't help with your question sorry. The only thing I can say is that when switching BCP women need to take them for at least 3 months before their body gets accustomed to it.