A method that has proven useful in predicting stacking behavior of androgens other than testosterone is to systematically group them as either “Class I” or “Class II.” Pairs of androgens which combine with little or no synergy are assigned to the same class. Androgens combining very synergistically are assigned to different classes.
The first assignment is arbitrary. Let us say that trenbolone is assigned to Class I. Then Primobolan (methenolone), Deca Durabolin (nandrolone decanoate), Equipoise (boldenone), and Anavar (oxandrolone) are also assigned to this class, since none of them act synergistically with trenbolone: a greater effect cannot be achieved by adding them than can be achieved with trenbolone alone at a sufficient dose.
As it happens, no combination of these Class I compounds proves much if at all synergistic, either, so the classification method is a logically consistent one.
Dianabol, on the other hand, combines very synergistically well with trenbolone acetate or other Class I compounds. It therefore is categorized as Class II. Other Class II pharmaceutical steroids include Anadrol and Winstrol.
It’s interesting to note that while both are Class II steroids, some synergy is observed between Winstrol and Dianabol. It is not, however, nearly as strong as the synergy between either of them and any Class I compound. While differing in some common way from all Class I compounds, in some cases Class II compounds may have some differences among themselves: Winstrol may have an activity that Dianabol for example does not, or possesses to a lesser extent.
It may be noted that where binding to the androgen receptor is known, those compounds that have good binding to the androgen receptor are invariably Class I, and those that do not (while having good anabolic effect regardless) are invariably Class II.
If one prefers, Class I steroids may be defined as those binding well to the androgen receptor, while Class II steroids are those which do not, yet have good anabolic effect.
Some discussion of the fact that androgens have more than one mechanism of action can be found in a previous article, Pharmacological Differences Between Anabolic Steroids. It is to be expected that where mechanisms of action may differ between drug compounds, combinations will yield synergy.