By adding an ester, the patient can visit the doctor as infrequently as once per month for his injection, instead of having to constantly re-administer the drug to achieve a therapeutic effect. Esterification temporarily deactivates the steroid molecule. In order for the compound to become active the ester must therefore first be removed. Now and only now will the steroid be able to have an effect on skeletal muscle tissue. You can start to see why considering testosterone cypionate much more potent than enanthate makes little sense, as your muscles are seeing only free testosterone no matter what ester was used to deploy it.
Esters vary only in their ability to reduce a steroid's water solubility.
An ester like propionate for example will slow the release of a steroid for a few days, while the duration will be weeks with a decanoate ester. Any differences in results and side effects that may be noted by bodybuilders who have used various esterified versions of the same base steroid are just issues of timing. Likewise testosterone suspension is the worst in regards to gyno and water bloat because blood hormone levels peak so quickly with this drug. Instead of waiting weeks for testosterone levels to rise to their highest point, here we are at most looking at a couple of days. Given an equal blood level of testosterone, there would be no difference in the rate of aromatization or DHT conversion between different esters.