That is the question.
etioallocholan = 5a-androst = ‘a skeleton’ or ‘a isomer’
etiocholan = 5b-androst = ‘b skeleton’ or ‘b isomer’
The information above has been distributed as part of some simplified steroidal nomenclature guides on internet forums. Many people have found that when they compare it to the nomenclature listed on some methasterone products (superdrol clones), they realise that if the labels on the products are accurate, that would mean the contents were some kind of 5b-isomer of methasterone.
This article will tell you exactly what a 5b-isomer is, and whether or not it’s true that some superdrol clones contain it.
A little knowledge is a dangerous thing
A few years ago people began to notice that certain brands of superdrol clones list the active ingredient as 2a,17a-dimethyl-etiocholan-3-one-17b-ol, believing this to indicate that the steroid inside is 5-b reduced version, or a “b-isomer” of superdrol. Many even go so far as to use this as an explanation for cycles with these drugs that do not meet the expectations of the user, citing the “weaker b-isomer” as the excuse for the poor results.
The etiocholan/etioallocholan information quoted is accurate – though relatively meaningless for many. A lot of people will be aware that some steroids are substrates of (can be affected by) the 5-alpha reductase enzyme, which removes the delta-4 double bond (turning testosterone into DHT, for example). Few people are aware that there is also a 5b reductase enzyme, which exists in the liver in greater quantities than 5a-reductase (in fact, testosterone reduces in the liver via 5b-R and 5a-R in the ratio 87:13 in favour of the 5b isomer). 
Although the difference between a 5a-reduced and 5b-reduced steroid is ostensibly only the position of a single hydrogen atom, that actually reflects a massive shift in the conformation of the structure.
These are simplified structural diagrams. On the left are the A and B rings of a 5a-androstane (etioallocholan) steroid, seen from above (top) and the side (below). On the right are the A and B rings of a 5b-androstane (etiocholan) steroid. From a chemistry standpoint, the 5a and 5b configurations can be described as trans and cis isomers, respectively.
Notice how the A-ring (left hand ring) of the 5a-reduced steroid is quite flat, in line with the rest of the rings, whereas the 5b-isomer is bent significantly? 5b-androstanes have a stereocenter at carbon 5 that results in the A ring being almost perpendicular to the rest of the molecule.
Lets examine what that means for androgen receptor binding, and therefore what effect it will have on the steroid’s effectiveness.
The steroids that are best at binding with the androgen receptor are those that are flattest. Taking methyltrienolone (also known as methyltrenbolone, metribolone, and R1881) for example, the conjugated double bonds on the steroid’s backbone make it extremely flat, and an excellent fit for the AR.  This is reflected in the anabolic and androgenic potency of the drug – it’s one of the strongest ever created.
The conformational change involved in 5b-reduction makes the steroid anything but flat, and an extremely poor fit for the receptor. By way of an example, 5a-DHT has 173 times stronger binding affinity than 5b-DHT.  5b-reduced steroids are effectively inactive.
The upshot being, if any superdrol clone was using a 5b-isomer of the methasteron molecule, it wouldn’t be “weaker”, it quite simply wouldn’t work.
So what do 5b steroids do?
If the definition quoted above about “etiocholan = 5b isomer” is so prevalent, being part of a “sticky” thread on nearly every bodybuilding forum with a prohormone/steroid section, it must have some significance, right? There must be some 5b steroids around. Well, there are, but they aren’t anabolic steroids.
Cholesterol is the starting point for everything from the steroid hormones (including testosterone) to Vitamins A, D, E and K, as well as maintaining cell membrane permeability and fluidity.
It’s also the source of the bile acids. When you eat, these acids are secreted into the intestine to help process dietary fats, and act as a sort of “natural pesticide”, killing off some of the microorganisms that live inside us to maintain a healthy gut flora.
D4–3-ketosteroid 5ß-reductase, to give 5b-reductase it’s proper title, serves two purposes. One is to effectively deactivate delta-4 sex steroids like testosterone as an intermediate step to excretion, and the other is in the conversion of cholesterol to bile acids.
This “5b isomer” rumour about certain brands of methasterone (superdrol) clones has persisted for years – despite several industry heavyweights lending their thoughts on the matter.
“To make the 5beta isomer would take an entirely different starting material that would be expensive and scarcely avaialable. I am not even sure if the reactions would go the same way if you started with the 5beta-androstane starting material (which would be something like etiocholanolone)
I would guarantee my nutsack that there has never been a 5beta-androstane version of superdrol on the market. All this issue ever was was confusion in nomenclature” 
“I think its more likely that this company doesnt know how to write the name correctly. I dont think anyone is making the 5b of superdrol, especially since superdrol raw material is easy to come by.” 
Serious Nutrition Solutions:
“We were the first company to label it as a 5b to the best of my knowledge. We were told that was the correct ‘technical’ way to state the compound on the label.
Ours was exactly the same as Superdrol. Testing confirmed it. However, it seems that at the time we were told to label it by the name we chose and that seems to be an error in which many other companies followed suit.” 
So now you know what etiocholan means, what a “b isomer” actually looks like, and (most importantly) why there’s never been a 5b isomer of superdrol.