Post Cycle Therapy (PCT), though beyond the scope of this profile, needs to be commented on. Due to the highly suppressive nature of Deca-Durabolin, I will speculate that testosterone in a Deca-Durabolin-inclusive cycle needs to be run for at least 2 additional weeks upon cessation of Deca. We remember from the chart above that baseline testosterone levels took roughly a month to return. Hence, a nice long estered testosterone should be run about 2 weeks longer than Deca-Durabolin, to prevent having a lag in time when the Deca-Durabolin is not producing an anabolic effect, yet is still suppressing your natural testosterone levels. Id also suggest that a particularly aggressive PCT be run after your cycle; nolvadex , HCG , and perhaps clomid should all be utilized in an effort to restore your natural hormone levels as quickly and efficiently as possible.
NPP is a nandrolone ester , or a prodrug of nandrolone .   As such, it is an androgen and anabolic steroid , or an agonist of the androgen receptor , the biological target of androgens like testosterone .   Relative to testosterone, NPP has enhanced anabolic effects and reduced androgenic effects.   In addition to its anabolic and androgenic activity, NPP has low estrogenic activity (via its metabolite estradiol ) and moderate progestogenic activity.  Like other AAS, NPP has antigonadotropic effects, which are due to both its androgenic and progestogenic activity. 
How do you protect yourself against this? Well, when taking Deca Durabolin you can also take ant-estrogen medications to counter the results. Things like Nolvadex and Arimidex are popular choices. Both approach limiting the effects of estrogen different and will have different side effects on your health worth considering. Even with side effects though it is still worth pursuing either of these options. Gynecomastia is a particularly big problem due to how the imbalance between estrogen and progesterone is what leads to gynecomastia. So, by keeping your hormone levels regulated, you reduce the risk of potential unwanted growth.