Below Is an Explanation of Deca-Dick, I received this info from Voxdeis, as he is very knowledgable.
I am going to attempt to make this as easy to understand as possible. It might take a couple readings to totally understand what is happening. There are three major reasons for the condition of "Deca Dick." Basically that condition is the inability to achieve a full and constant erection and a significant decrease is sexual drive and pleasure. The three main reasons why 19nortestosterone/Deca causes these are, decrease in DHT production below necessary levels, decrease or total inhibition of LH (Leutinizing Hormone) production, and lastly a stopping of GnRH (Gonadotropin Releasing Hormone.)
DHT is androgen that works on many parts of the body. It is created by the conversion of testosterone in the prostrate by the enzyme 5-alpha-reductase. DHT plays a major roll in testicular functioning and health, and therefore sperm and testosterone production. The nortestosterones like Deca do not convert to DHT and therefore that will not aid in the process of testicular health. However, nortestosterones, more than any other androgen, will stop natural testosterone production.
How does nortestosterone do this? It has the ability to bind tightly to receptors in the brain that shut off GnRH production and therefore limiting FSH (Follicle Stimulating Hormone) and LH to some degree. Nortestosterones also convert to estrogen rapidly and therefore the higher estrogen will bind to the hypothalamus, a part of the brain involved in LH output which LH is responsible for triggering the testicles to produce testosterone. Why one takes clomid is to block the estrogen from shutting that systematic testosterone production loop down.
The decreased erectile performance comes from this multilevel shutting down of the testicles. Sexual desire is usually associated with the increased level of cortisol in relationship to the levels of catecholamines in the brain. The catecholamines are the good feeling brain chemicals and when cortisol exceeds a certain level those chemicals start to decrease. Deca, because of its high binding affinity also raises cortisol levels very high, therefore decreasing sexual desire over time. It takes a long time for cortisol levels to stabilize if left untreated.