However, its not all good I’m afraid. Deca is known to cause severe and rapid testosterone suppression even after doses of 100mg. After administration of only 100mg Deca Durabolin, endogenous testosterone was hypogondal levels for a full 4 weeks before returning to baseline. That means Deca should never be run alone and always in combination with other testosterone based anabolics. ‘Deca dick’ is a common side effects, when at leats a replacement dose of testosterone isn’t used. Erectile dysfunction can also be evident in users.