How Do Anabolic Substances Work? - Episode 5: Pharmacokinetics: Absorption, Distribution, Metabolism and Elimination

1. What is pharmacokinetics and why does it matter?

Pharmacokinetics is the story of a substance's journey through the body. Instead of asking only what the molecule does to the body, pharmacokinetics asks what the body does to the molecule. For an androgen, this means four broad processes: absorption, distribution, metabolism, and elimination, often summarized as ADME.

This distinction is essential. Two molecules that act on the same molecular target can have very different profiles if one is absorbed rapidly and the other slowly, if they distribute differently, or if their metabolism produces different metabolites. In pharmacology, the molecule and its journey are two chapters of the same story.

2. Pharmacokinetics versus pharmacodynamics

Pharmacodynamics describes what a substance does to the body, while pharmacokinetics describes what the body does to the substance. If the androgen receptor is the center of the effect, pharmacokinetics is the map showing how much of the signal reaches tissue, when it arrives, and how long it remains available.

This distinction explains why a drug with the same molecular target can behave differently depending on formulation, route of administration, and exposure over time.

3. Absorption: the first barrier

Absorption is the movement of a substance from its site of administration into the systemic circulation. With an oral compound, the substance must cross the gastrointestinal tract and then pass through the liver before a substantial fraction reaches the general circulation. Other routes follow different paths.

The speed and extent of absorption depend on the molecule, formulation, biological environment, and route. That is why the same substance does not necessarily create the same blood-concentration profile over time.

4. Bioavailability: how much actually reaches the blood?

Bioavailability describes the fraction of an administered substance that reaches the systemic circulation. With intravenous administration, bioavailability is complete. With other routes, some of the substance can be lost through incomplete absorption or metabolism before it reaches the systemic circulation.

For orally administered androgens, first-pass metabolism can be important. Some molecules have been structurally modified to survive this barrier better, and that same modification can change both systemic exposure and the safety profile.

5. Hepatic first-pass metabolism

After gastrointestinal absorption, portal blood carries many substances to the liver. Hepatic enzymes may transform part of the molecule before it enters the systemic circulation. This is called first-pass metabolism.

For androgen pharmacology, this principle matters because resistance to first-pass metabolism can increase oral availability. Reviews of AAS describe rapid gastrointestinal absorption for some molecules and a major role for hepatic metabolism in determining subsequent systemic exposure. [Source: PubMed, AAS pharmacokinetics review]

6. Why chemical structure changes pharmacokinetics

In medicinal chemistry, relatively small structural changes can alter solubility, absorption, metabolic stability, protein binding, and half-life. In testosterone pharmacology, esterification is a classic example of a structural change that can alter release and duration of exposure.

This does not mean an esterified form is a completely different hormone at the receptor. In many injectable preparations, the ester mainly changes physicochemical properties and release rate, after which esterases release the active hormone.

7. Injectable depot and slow release

Some injectable formulations create a depot at the site of administration from which the substance is released gradually. In this situation, the rate of absorption from the depot can become the process that limits how quickly the substance reaches the circulation.

Pharmacokinetic studies of androgen esters show that ester structure influences absorption rate and duration of exposure. Slower release from the depot can create a more prolonged profile. [Source: PubMed, Pharmacokinetics of anabolic steroids]

8. Plasma concentration is not constant

After administration, blood concentration follows a time curve. There may be a rising phase, a peak, and then a decline. The exact shape depends on absorption, distribution, metabolism, and elimination.

A profile with rapid peaks and declines does not create the same biological exposure as a more stable profile. That is why maximum concentration alone is not enough to compare formulations.

9. Cmax, Tmax, and AUC

Cmax is the maximum observed concentration, while Tmax is the time needed to reach it. AUC, the area under the concentration-time curve, represents total systemic exposure over a defined period.

For a non-specialist reader, these concepts can be translated into three simple questions: how high does the signal rise, how quickly does it get there, and how much total exposure occurs over time?

10. Distribution: where does the substance go?

After absorption, molecules are transported in blood and distribute between compartments of the body. For lipophilic steroids, physicochemical properties and protein binding influence tissue distribution.

Blood is not the final destination. It is the transport system. A molecule can leave plasma, enter tissues, cross cell membranes, and reach cells where it is metabolized or interacts with the androgen receptor.

11. Binding to plasma proteins

Testosterone circulates in bound and unbound forms. SHBG and albumin participate in transport and influence hormone availability. The dynamics are more complicated than the simple phrase free versus bound.

Protein binding changes distribution and can influence how long a molecule remains in circulation. That is why total concentration is not the only relevant information when discussing androgens.

12. Tissues are not simple reservoirs

A tissue can modify an androgen before receptor interaction occurs. Testosterone can be locally converted to DHT through 5-alpha-reductase or to estradiol through aromatase. Distribution and metabolism therefore connect directly to pharmacodynamics.

This is one of the key ideas of the series: the same concentration in blood does not guarantee the same biological exposure in every tissue.

13. Metabolism: the body changes the molecule

Metabolism, or biotransformation, changes the chemical structure of a substance. For many androgens, the liver is a major site of these transformations, but metabolism can also occur in the kidneys and peripheral tissues.

The biological goal is often to transform lipophilic molecules into products that are easier to conjugate and eliminate. Metabolism does not always mean simple inactivation, because active metabolites can be produced.

14. Phase I: the first transformations

Phase I reactions include oxidation and reduction. In androgen metabolism, these reactions can modify positions on the steroid nucleus and prepare molecules for later reactions.

The AAS literature describes reductions, hydroxylations, and other changes to the steroid molecule, often followed by conjugation. [Source: PubMed, Metabolism of anabolic androgenic steroids]

15. Phase II: conjugation

In phase II reactions, metabolites can be conjugated, for example with glucuronic acid or sulfate. These changes usually increase polarity and facilitate transport toward elimination pathways.

For AAS, metabolite conjugation is important in anti-doping science because many compounds are detected indirectly through their transformation products.

16. Metabolites are not just chemical waste

Some metabolites are inactive, but others remain biologically active. DHT and estradiol are important examples in testosterone physiology. The body therefore does not simply remove the molecule; it can transform one biological signal into another.

This can explain why the effects of an androgen are not always easy to predict from the parent molecule alone.

17. Elimination: how does the substance leave the body?

Elimination is the removal of the parent compound and its metabolites from the body. For androgens, the kidneys and the biliary-intestinal system are important routes, depending on the molecule and metabolite.

Urine and feces can contain conjugated metabolites or other degradation products. Some compounds or metabolites may also undergo enterohepatic recycling, which can prolong persistence of certain products.

18. Half-life

Half-life is the time required for plasma concentration to fall by about half in a given pharmacokinetic model. It is useful for describing disappearance from the circulation, but it does not by itself define the duration of every biological effect.

With a drug that is absorbed slowly from a depot, the concentration profile observed in blood may be influenced more by absorption than by elimination. That is why half-life sometimes requires context.

19. Pharmacokinetics and androgen esters

In some preparations, esterification changes physicochemical properties and slows release from the depot. Once the ester reaches the circulation, esterases can release the active hormone.

Pharmacokinetic literature shows that depot absorption can become rate-limiting and that different ester structures can create different durations of exposure. [Source: PubMed, Pharmacokinetics of anabolic steroids]

20. Oral versus injectable: different pathways

Oral and injectable routes change the pharmacokinetic path. Orally, gastrointestinal absorption and first-pass metabolism are central. For some injectables, slow depot absorption is the key determinant.

Two products that ultimately deliver the same active substance can therefore produce different peaks, troughs, and cumulative exposure. In pharmacology, the identity of the molecule is only the first question.

21. The transdermal route

Through the skin, absorption can bypass much of the gastrointestinal and hepatic first-pass pathway. But absorption is influenced by skin properties, application site, and formulation.

For transdermal testosterone, the application site can change systemic levels and local conversion to DHT. Recent research on testosterone formulations shows that anatomy and formulation can materially alter the pharmacokinetic profile.

22. Volume of distribution

Apparent volume of distribution is a parameter used to describe how a substance is distributed between blood and tissues. It is not a literal anatomical volume. A larger apparent volume may indicate more extensive distribution outside plasma in a given model.

For a general reader, the idea can be simplified this way: the more a molecule leaves the bloodstream and distributes into tissues, the less plasma concentration alone may represent the total amount present in the body.

23. Clearance: the rate of removal

Clearance describes the apparent volume of plasma from which a substance is removed per unit of time. It is influenced by hepatic, renal, and other elimination processes when relevant.

Clearance and volume of distribution contribute to the time course of a drug. In a simplified model, together they help explain half-life.

24. Pharmacokinetics and the liver

The liver is not merely a filter. It is a metabolic center. Some oral AAS have properties that allow them to resist first-pass metabolism, and that can increase oral exposure while also changing the hepatic safety profile.

Medical literature describes substantial hepatic metabolism of AAS and the fact that certain steroid classes can cause liver injury. Pharmacokinetics cannot be completely separated from toxicology.

25. Pharmacokinetics and anti-doping detection

In anti-doping testing, laboratories do not always look only for the original molecule. Many AAS are extensively metabolized and can be detected through characteristic metabolites or metabolic profiles.

Some detection strategies rely on the fact that metabolites can remain identifiable after the parent concentration has fallen substantially. This is a practical example of why chemical elimination and analytical detectability are not the same thing.

26. Detectability is not the same as biological effect

A metabolite may be detectable in a sample when the main pharmacological effect is no longer maximal. Conversely, a low concentration in blood does not mean every biological effect disappears instantly.

That is why pharmacokinetics for treatment, pharmacokinetics for effect, and pharmacokinetics for detection can ask different questions about the same molecule.

27. Accumulation

When exposure is repeated before a substance has been largely eliminated, concentrations can overlap from one administration to the next. Accumulation then occurs until input and elimination reach a dynamic balance.

This explains why a concentration measured on one day cannot be interpreted without considering exposure history and the substance's pharmacokinetic properties.

28. Steady state

With repeated administration, a drug may approach steady state, an average condition in which input and elimination balance over time. The time needed depends mainly on half-life and the pharmacokinetic model.

Formulations with very slow absorption or depot mechanisms can have more complicated profiles than a simple model with immediate absorption and rapid elimination.

29. Variability between people

Two people can have different concentrations and different responses to the same molecule. Body size, body composition, hepatic and renal function, genetics, age, other medicines, and formulation characteristics can all change pharmacokinetics.

In an athlete with very high body mass, distribution and volume of distribution may have particular features. This does not mean pharmacokinetics follows a simple rule such as more mass equals the same exposure in a different form. Real biology is more nuanced.

30. Why pharmacokinetics matters for risk

Risk depends not only on which receptor is activated, but also on how long tissues are exposed and at what level. A substance that creates prolonged exposure may have a different risk profile from one producing rapid fluctuations even when the molecular target is similar.

Pharmacokinetics is the link between molecule and time. Without it, discussion of effects and adverse effects is incomplete.

31. What should an athlete remember about ADME?

Absorption tells us how the substance enters the circulation. Distribution tells us where it goes. Metabolism explains what it becomes. Elimination shows how it leaves the body. Together, these four processes build the exposure curve.

For AAS, that curve can be influenced by chemical structure, route of administration, protein binding, local enzymes, liver, kidneys, and tissues. There is therefore no single pharmacokinetic signature shared by all steroids.

32. Conclusion: a molecule has a journey, not just a target

In the previous episodes we followed the androgen receptor, testosterone, and the distinction between anabolism and androgenism. Now we have the piece that connects them through time: pharmacokinetics. An androgen has to enter the body, be absorbed, distributed, metabolized, and eliminated.

Understanding ADME helps explain why the same family of molecules can have very different profiles and why chemical structure, route of administration, and metabolism matter so much. Pharmacology is not only the receptor receiving a signal. It is the complete story of the signal's journey through the body.

Selected references

This episode draws on review literature covering anabolic-androgenic steroid pharmacokinetics, androgen metabolism, testosterone distribution, androgen esters, and metabolite detection in anti-doping science.