Here is a great video for you to watch where I explain what deca dick is, and how you can prevent it ->
http://www.evolutionary.org/forums/anabolic-steroids-peds/new-video-what-deca-dick-48296.html
Deca dick is the result of high estrogen/prolactin levels AND/OR from not including an androgenic steroid in your steroid cycle.
Deca is a nandrolone which increases prolactin levels in your body. When you run a cycle with deca, you need include both:
An AI like aromasin - to keep estrogen in the low-normal range
An anti prolactin drug like cabergoline -to keep prolactin in the low-normal range
Deca is primarily an anabolic steroid .The problem with that is that if you run deca by itself, your natural testosterone production will be suppressed and you can experience low androgen side effects like low libido. You always want to include an androgenic steroid into a cycle with deca like testosterone, proviron, or trenbolone.
I personally feel stacking deca with testosterone makes the most sense.
In summary, you can prevent deca dick by:
1. Running your testosterone dosage higher then the deca. I.e. testosterone at 500mg/week and deca at 300mg/week
2. Use an AI with your cycle to keep estrogen in the low-normal range - aromasin at 10mg EOD
3. Use an anti-prolactin drug to keep prolactin in the low-normal range - cabergoline at 0.25mg EOD