Nolvadex and/or arimidex may help prevent gyno from Deca. Many people either are not aware, or forget, that Deca is a substrate for aromatase and can be converted to estrogen (1) (2). Obviously they will not prevent any possible effects progesterone/prolactin might have on causing gyno.
(1) J Steroid Biochem 1989 Nov;33(5):949-54
Androgen and 19-norandrogen aromatization by equine and human placental microsomes.
Dintinger T, Gaillard JL, Moslemi S, Zwain I, Silberzahn P.
(2) 1: Nippon Naibunpi Gakkai Zasshi 1986 Jan 20;62(1):18-25
[Aromatization of androstenedione and 19-nortestosterone in human placenta, liver and adipose tissues]
Yoshiji S, Yamamoto T, Okada H.