Vitex Vs. Soy Isoflavones?

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I know they can't be taken together, but I'm wondering which is better? Does anyone have an opinion one way or the other? I'm currently having 35-38 day cycles, and I don't think I'm ovulating. I have not gotten a positive OPK in 3 cycles now. Going to chart BBT next cycle when I start taking whichever supplement I choose. Love to hear any info!