Anabolic-androgenic steroids (AAS) are known for their ability to increase muscle size and strength, which is why they are associated with bodybuilding and strength sports. But how does building muscle with anabolic steroids actually work, what does controlled research show, and what are the trade-offs?
This article explains the biological mechanisms behind AAS-related muscle growth, what studies have found about dose and training, how different compounds differ, and the health risks that accompany these effects. For a broader introduction, see our guide to the key facts about steroids.
How do anabolic steroids build muscle?
AAS are testosterone and synthetic compounds related to it. Their effects on muscle come mainly from activating androgen receptors, although several connected processes are involved.
Androgen receptor activation
AAS bind to androgen receptors inside muscle cells. Once activated, these receptors influence the activity of genes involved in building muscle tissue. The result is a shift in the balance between muscle protein synthesis and muscle protein breakdown toward net growth.
Compounds differ in how strongly they produce tissue-building effects relative to male-typical effects. Our article on anabolic vs androgenic effects explains why that distinction is less clear-cut in practice than it is often presented.
Satellite cells and myonuclei
Muscle fibers do not multiply by dividing. Instead, they grow larger, and growing fibers need additional nuclei to support the extra tissue. These nuclei come from satellite cells, the stem cells that sit alongside muscle fibers.
Research in healthy young men found that testosterone-induced muscle growth was associated with an increase in satellite cell numbers (Sinha-Hikim et al., 2003). Studies of strength-trained athletes using AAS have also found larger muscle fibers with more nuclei than in non-users (Kadi et al., 1999).
Other contributing effects
AAS may also reduce muscle protein breakdown by interfering with the effects of cortisol, a hormone that promotes tissue breakdown. In addition, they increase red blood cell production. This is sometimes presented as a performance benefit, but it also raises the risk of erythrocytosis, an abnormally high red blood cell count, which is discussed in the risks section below.
What does research show about steroids and muscle growth?
The muscle-building effects of AAS are well established in controlled research.
Muscle growth can occur even without training
In a landmark study, healthy men received a supraphysiological dose of testosterone or a placebo, with or without resistance training, for 10 weeks. Testosterone increased muscle size and strength even in men who did not train, and the largest increases occurred in men who combined testosterone with training (Bhasin et al., 1996).
This shows that AAS act directly on muscle, but also that training substantially amplifies the response.
Effects are dose-dependent
A later study gave men different doses of testosterone while suppressing their own production. Gains in lean mass and strength increased with dose. However, adverse effects also increased with dose, including reductions in HDL cholesterol (Bhasin et al., 2001).
This dose-dependence applies to both the effects people seek and the harms they want to avoid. Higher exposure does not separate the two.
More muscle does not automatically mean proportional strength
Strength also depends on the nervous system, skill, and how strength is measured. As explained in our article on muscle size vs strength, size gains and strength gains do not always move together.
How do different anabolic steroids differ?
AAS vary in chemical structure, and these differences affect how they are absorbed, how long they act, and which side effects are most prominent.
Injectable steroids
Most injectable steroids are attached to an ester, a chemical group that controls how quickly the compound is released into circulation after injection. Longer esters produce slower release and longer activity, while shorter esters act more quickly.
Oral steroids
Many oral steroids are chemically modified, most commonly through 17-alpha-alkylation, so that they survive processing by the liver after being swallowed. This modification is also the main reason oral compounds are associated with greater liver strain and more pronounced reductions in HDL cholesterol than many injectable compounds.
No compound is “mild” in absolute terms
Some compounds are commonly described as mild or as having fewer side effects. These descriptions usually reflect relative comparisons between compounds, not an absence of risk. All AAS can suppress natural testosterone production, and the overall risk depends on the compound, dose, duration of use, and the individual.
The role of training and nutrition
Training and nutrition remain the main drivers of muscle growth for everyone, with or without AAS. The research described above found that testosterone produced substantially greater gains when combined with resistance training (Bhasin et al., 1996).
The principles of muscle growth, including progressive overload, sufficient training volume, adequate protein and energy intake, and recovery, are the same for people who use AAS and those who do not. AAS do not replace these factors, and people who make good progress without them often underestimate how much training and nutrition contribute.
What are the health risks?
The same hormonal activity that drives muscle growth also affects many other systems in the body. The risks depend on the compounds, doses, duration, and individual factors, and much of the evidence comes from observational studies.
Hormonal suppression
AAS suppress the brain signals that drive the body’s own testosterone production. After use stops, natural production may take months or longer to recover, and in some people it remains low. Our article on testosterone recovery after steroids explains this process and why timelines vary.
Cardiovascular effects
AAS use is associated with increases in blood pressure, unfavorable changes in blood lipids, and changes in heart structure. Our article on how steroids affect cholesterol levels covers the lipid evidence, including the reduction in HDL cholesterol that is one of the most consistent findings.
Increased red blood cell count
The increase in red blood cell production caused by AAS can lead to erythrocytosis. Thicker blood may increase the risk of blood clots and other cardiovascular problems.
Liver effects
Oral AAS, particularly 17-alpha-alkylated compounds, are associated with liver strain and, more rarely, serious liver conditions. There is no good evidence that over-the-counter supplements reliably prevent AAS-related liver injury, so they should not be considered a safeguard.
Fertility and other effects
AAS can sharply reduce sperm production, and they have also been associated with acne, breast tissue development in men, mood changes, and dependence. In women, they can cause virilization, including voice deepening that may be permanent.
What happens to muscle after stopping steroids?
When AAS use ends, some of the muscle gained during use is typically lost, particularly as hormone levels fall and if natural testosterone production remains suppressed. However, the extent of loss varies, and some research suggests that myonuclei gained during growth may persist for some time.
Our article on what happens to muscle after stopping steroids explains what the evidence shows and why muscle loss is not the same as losing all steroid-associated gains.
Legal status
In the United States, anabolic steroids are Schedule III controlled substances and can be legally obtained only with a valid prescription. In competitive sport, they are prohibited at all times under the World Anti-Doping Agency Prohibited List.
The bottom line
Building muscle with anabolic steroids works through well-understood mechanisms: androgen receptor activation, increased muscle protein synthesis, and satellite cell activity that supports larger muscle fibers. Controlled research shows that AAS can increase muscle size and strength, that the effects are dose-dependent, and that training substantially amplifies the response.
The same research shows that adverse effects also rise with dose. AAS affect hormonal, cardiovascular, liver, reproductive, and psychological health, and no compound or supplement makes them risk-free. Anyone with questions about past or current AAS use, or its effects on their health, should speak with a qualified healthcare professional.
References
Bhasin S, Storer TW, Berman N, et al. The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men. New England Journal of Medicine. 1996;335(1):1–7.
Bhasin S, Woodhouse L, Casaburi R, et al. Testosterone dose-response relationships in healthy young men. American Journal of Physiology: Endocrinology and Metabolism. 2001;281(6):E1172–E1181.
Kadi F, Eriksson A, Holmner S, Thornell LE. Effects of anabolic steroids on the muscle cells of strength-trained athletes. Medicine & Science in Sports & Exercise. 1999;31(11):1528–1534. Sinha-Hikim I, Roth SM, Lee MI, Bhasin S. Testosterone-induced muscle hypertrophy is associated with an increase in satellite cell number in healthy, young men. American Journal of Physiology: Endocrinology and Metabolism. 2003;285(1):E197–E205.
