How Anabolic Steroids Affect Your Immune System and Infection Risk

Anabolic steroid users were nearly five times more likely to contract COVID-19 than non-users in a 2022 MDPI study of 520 gym-attending participants, and those who did were significantly more likely to experience moderate to severe disease. That single finding captures something the bodybuilding community rarely discusses honestly: steroids do not leave your immune system untouched. The effect is compound-specific, dose-dependent, and more nuanced than a simple suppression story, but the direction is clear. On a heavy cycle, your ability to fight infections changes. This blog covers exactly how, which compounds carry the most risk, and what it means for how you manage illness when you are on.

Anabolic Steroids Do Not Have One Uniform Effect on Immunity

Most people assume steroids simply suppress the immune system. The research is more specific than that, and getting the detail right changes which compounds you should worry about.

Research published in the Central European Journal of Biology confirmed that supraphysiologic doses of AAS with an intact steroid nucleus are immunosuppressive, reducing immune cell number and function. Those with structural alterations to the steroid nucleus behave differently, inducing the proliferation of T cells and other immune cells instead.

Testosterone, nandrolone, and trenbolone all have an intact steroid nucleus. At bodybuilding doses, they suppress lymphocyte counts and reduce adaptive immune activity. Stanozolol and oxandrolone have structural alterations. A PubMed study testing all five compounds found that while all groups showed suppression at day five, the stanozolol, oxandrolone, and testolactone groups shifted to immune stimulation by day ten.

The immune effect is not fixed at the start of a cycle. It changes over time, and it differs meaningfully by compound.

Which Immune Cells Are Affected and How?

The immune system is not one thing. Steroids affect different cell types in different directions.

Lymphocytes (T cells, B cells, Natural Killer cells) are your adaptive immune system. They recognize specific pathogens, produce antibodies, and run the targeted response to infections. Research confirms that common AAS adversely influence lymphocyte differentiation, proliferation, antibody production, and Natural Killer cell cytotoxic activity. High testosterone levels have been associated with decreased lymphocyte production, which directly weakens the adaptive response to new infections.

Neutrophils are your first responders. They arrive at infection sites immediately and destroy pathogens by engulfing them. Testosterone enhances neutrophil activity. This is partly why steroid users do not collapse with every minor illness. The front-line defense is not weakened. The more sophisticated adaptive response is.

Natural Killer cells present a more complex picture. A Nature 2024 study found that NK cells showed stronger interferon responses (the signaling protein that tells nearby cells a virus is present) following three months of testosterone treatment, driven by improved IL-12 activity in monocytes. This suggests that while adaptive lymphocyte function declines, certain NK cell behaviors may actually improve under testosterone exposure.

The net effect: your immediate and innate defenses hold up reasonably well. Your ability to mount a targeted adaptive response to unfamiliar pathogens is where the suppression is most meaningful.

How Testosterone Actually Suppresses Immunity: The Stanford Finding?

Stanford University researchers found something that changes how steroid-related immune suppression should be understood.

The study found no direct suppression of immune cells from circulating testosterone. Instead, testosterone interacts with a specific set of genes in a way that dampens immune signaling, according to the study’s senior author Professor Mark Davis. The suppression is indirect, working through gene expression rather than directly destroying immune cells.

A separate redistribution hypothesis adds another layer. One research group proposed that testosterone signals infection-fighting white blood cells to move out of the bloodstream and into skin tissue, meaning a blood panel showing low white blood cell counts in a high-testosterone male may reflect redistribution rather than true depletion.

If redistribution is occurring, standard blood test white blood cell counts on cycle may look more alarming than the actual immune situation warrants. It does not eliminate the concern. Cells in skin tissue are not circulating where systemic infections need them. But it does complicate the interpretation of WBC counts on a blood panel.

The COVID-19 Data: What Real-World Infection Looks Like

The COVID-19 pandemic provided the most significant real-world dataset on AAS users and infection outcomes ever collected.

The 2022 MDPI cross-sectional study of 520 gym-attending participants found that current AAS users contracted COVID-19 at 35.90% versus 18.92% for non-users. Multivariable analysis, controlling for other factors, put the odds of contraction at nearly five times higher for current users. Current AAS use was also independently associated with greater odds of moderate to severe disease progression.

A separately published PubMed case report documented severe COVID-19 in an otherwise healthy 28-year-old bodybuilder taking oxandrolone 40mg daily. Symptoms deteriorated rapidly over four days, prompting the authors to describe it as the first reported case potentially linking AAS use to COVID-19 severity.

Two honest caveats: this COVID data comes from observational studies, not controlled trials. Steroid users in these populations may differ from non-users in other lifestyle factors. The five-times figure reflects association, not proven causation. The mechanistic research makes the association biologically plausible, but the causation is not established beyond doubt.

How Overtraining Compounds Immune Suppression on Steroids?

Steroid use and heavy training do not cancel each other out. They stack.

High-intensity training independently reduces the number and activity of natural killer cells and T cells. It also lowers IgA levels in saliva, which is the primary mucosal defense against respiratory pathogens entering through the mouth and throat. When salivary IgA drops, the mucous membranes become more permeable to infection. A steroid user training at high volume with inadequate recovery is running two simultaneous immune suppression mechanisms at once.

This explains why steroid users who overtrain report significantly more frequent upper respiratory infections than those who train hard but manage recovery properly. The illness frequency is not purely a steroid effect. It is a steroid-plus-overtraining effect.

Which Steroids Carry the Most Immune Risk

Higher immunosuppression risk (intact steroid nucleus):

• Testosterone (all esters): Suppresses lymphocyte counts and antibody production at bodybuilding doses while enhancing neutrophils
• Nandrolone (NPP, Deca): Similar immunosuppressive profile, also suppresses natural testosterone production
• Trenbolone: High androgenicity, intact nucleus, consistent with immunosuppressive profile at bodybuilding doses

Lower or mixed immune effect (altered steroid nucleus):

• Stanozolol (Winstrol): Showed immunostimulatory shift by day ten in the Mendenhall study
• Oxandrolone (Anavar): Also showed immunostimulatory shift, lower immunosuppression risk relative to testosterone-based compounds

Lower risk does not mean no effect. All anabolic steroids alter immune function in some direction.

Should You Get Vaccinated While on a Steroid Cycle?

The adaptive immune cells most responsible for generating antibody responses to vaccines, specifically B cells and T cells, are among those suppressed by intact-nucleus AAS at bodybuilding doses.

No controlled trial has directly compared vaccine antibody responses in on-cycle versus off-cycle steroid users, so this cannot be stated as a research-confirmed fact. But the mechanistic inference is reasonable: reduced lymphocyte function during a heavy testosterone cycle may produce a blunted antibody response to vaccination compared to an off-cycle baseline.

Where timing vaccinations for off-cycle periods is practically possible, it is the more cautious approach based on what the mechanism suggests. Where it is not possible, getting vaccinated on cycle is still better than not getting vaccinated.

What to Do When You Get Sick on a Cycle

Two questions every steroid user has when they get ill but almost no content directly answers.

Will my illness be worse on cycle than off? 

For common upper respiratory infections, the evidence suggests illness may last longer and be harder to shake on a heavy intact-nucleus cycle, because the adaptive immune response driving recovery is partially suppressed. The COVID data also points toward more severe outcomes in steroid users. This does not mean every cold becomes dangerous, but treating illness more seriously on cycle than off is the evidence-consistent approach.

Should you stop the cycle when you get sick? 

There is no clinical trial answer to this. The practical considerations are: the steroid itself is not treating the infection and may be slowing recovery by suppressing adaptive immunity. Continuing a heavy cycle through a significant infection, particularly one involving high fever, respiratory symptoms, or systemic illness, adds an immunosuppressive load to a system already under stress. Reducing dose or pausing an oral compound during significant illness is a reasonable precaution. Continuing to train hard through illness on cycle is not.

Other practical points:

• Do not train through illness. Overtraining and steroid-related immune suppression stack badly
• Tell your doctor what compounds you are running if you seek medical treatment. It is medically relevant information
• Minor colds may not warrant cycle changes. Significant infections with systemic symptoms are a different situation

FAQs

Do anabolic steroids suppress the immune system? 

Intact-nucleus steroids like testosterone, nandrolone, and trenbolone suppress lymphocyte count and adaptive immune function at bodybuilding doses. Structurally altered compounds like stanozolol and oxandrolone showed immunostimulatory effects in research. No anabolic steroid leaves immune function entirely unchanged, but the direction and degree differ significantly by compound.

Do steroids make you more susceptible to infections? 

Yes, based on available evidence. A 2022 study found current AAS users were nearly five times more likely to contract COVID-19 than non-users. Upper respiratory tract infections are reported more frequently by steroid users, particularly those training at high volume with inadequate recovery alongside the cycle.

Which steroids are most immunosuppressive? 

Testosterone, nandrolone, and trenbolone carry the highest immunosuppressive risk based on their intact steroid nucleus structure and the research on lymphocyte suppression. Stanozolol and oxandrolone showed a shift toward immune stimulation in research, suggesting a lower immunosuppressive profile relative to testosterone-based compounds.

Should you stop your steroid cycle if you get a serious infection? 

No controlled trial directly answers this. The reasonable approach is to consider reducing dose or pausing oral compounds during significant illness with systemic symptoms, since the cycle adds immunosuppressive load to a system already fighting infection. Minor illnesses may not warrant stopping. Significant infections with high fever or respiratory involvement are a different situation worth discussing with a doctor.

Conclusion

Anabolic steroids change how your immune system functions, and the change is not trivial. Intact-nucleus compounds suppress the adaptive immune cells most responsible for fighting unfamiliar infections and generating vaccine responses. Real-world COVID-19 data shows the susceptibility increase translates into meaningfully worse outcomes in the population, not just on paper. Overtraining on top of a cycle compounds both risks simultaneously. Understanding which compounds carry the most immunosuppressive risk, what to do when illness hits during a cycle, and how to think about vaccination timing gives you practical tools that most steroid users never have access to because the topic is almost never covered with this level of specificity.

Disclaimer: This article is for informational and educational purposes only. It does not constitute medical advice. Anabolic steroids are controlled substances in many countries and carry serious health risks. Always consult a qualified medical professional regarding any health concerns, particularly during illness.