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Science & Education

rbST: Know the Science.

You've heard the claims. Here are the questions, the evidence, and the sources — including the uncomfortable ones.

Explore the questions Check the sources
New in 2025

30 years of somatotropin science, revisited.

Invited review · Journal of Dairy Science, July 2025 · open access

A review of decades of published evidence, not a new field trial — and its authors disclose past industry relationships. We link the paper so you can weigh it yourself.

Read our summary Open the paper ↗
  1. 01Milk production: rbST increases milk yield through coordinated metabolic and physiological adaptations.
  2. 02Feed efficiency: the authors report improved feed efficiency alongside the higher yield.
  3. 03Animal health: well-managed dairies using rbST showed no adverse effects on health and well-being compared with appropriate controls.
  4. 04Sustainability: greater productive efficiency can reduce resource use and environmental impact per unit of milk.
Our approach

Ask. Explain. Source.

The strongest science communication does not hide difficult evidence. It separates consumer safety, animal health, regulation and farm management — then lets readers inspect the source.

  1. 01
    Ask the real question.

    Use the language people actually search: cancer, hormones, mastitis, antibiotics, Europe, welfare.

  2. 02
    Answer with nuance.

    No blanket “myth busted” claims where studies or jurisdictions genuinely differ.

  3. 03
    Show the evidence.

    Primary regulator pages first. Peer-reviewed literature adds depth and context.

  • U.S. statusFDA approvedsince 1993
  • Food safetyJECFAreassessed in 2013
  • EU decisionAnimal welfarekey policy rationale
  • Our ruleLink the sourcedon't ask for trust
Explore by topic

Start where your question starts.

Hard questions first

No topic is off limits.

These are the questions most likely to shape perception — and the ones that deserve the clearest sourcing.

Human Safety

Does milk from rbST-treated cows cause cancer?

Regulatory reviews have not established that consuming milk from rbST-treated cows increases cancer risk.

Read the evidence
Human Safety

What about IGF-1 in milk?

IGF-1 is naturally present in milk and in the human body. The key question is whether dietary exposure from milk meaningfully changes human biological exposure.

Read the evidence
Animal Health

Does rbST increase mastitis?

Some studies and FDA’s original evaluation found an increased risk of clinical mastitis; later meta-analyses using the U.S.-approved formulation and label conditions reported different results.

Read the evidence
Milk & Antibiotics

Does rbST mean more antibiotics in the milk supply?

FDA’s post-approval review found no product-related increase in violative antibiotic residues in the U.S. milk supply.

Read the evidence
Regulatory Context

If rbST is safe for people, why is it prohibited in the European Union?

The EU’s 1999 decision focused on animal-health and animal-welfare concerns, not a finding that milk from treated cows was unsafe for human consumption.

Read the evidence
Basics

Does using rbST make the cow or its milk genetically modified?

No. Recombinant DNA technology is used to manufacture rbST; it does not genetically modify the treated cow or the milk.

Read the evidence
Evidence library

Questions about rbST.

Search or filter the full library. Every answer links to the underlying regulatory or scientific reference.

What is rbST — and how is it different from bST?Basics

bST is a protein hormone naturally produced by cattle. rbST is a recombinant version produced with biotechnology and used as a dairy production aid.

The important distinction is between the naturally occurring bovine somatotropin (bST) and recombinant bovine somatotropin (rbST). Posilac contains sometribove zinc, an rbST formulation. FDA describes bST as a large protein hormone and Posilac as an FDA-approved animal drug for use in healthy lactating dairy cows according to its label.

Sources
Link to this answer
Is rbST a steroid?Basics

No. bST/rbST is a protein hormone, not a steroid hormone.

Calling rbST a “steroid” is chemically inaccurate. Somatotropin is a protein/peptide hormone. That distinction matters because proteins are handled very differently by digestion and by receptors in the body.

Sources
Link to this answer
Does using rbST make the cow or its milk genetically modified?Basics

No. Recombinant DNA technology is used to manufacture rbST; it does not genetically modify the treated cow or the milk.

The word “recombinant” describes how the protein is produced. Administration of a recombinant protein does not alter the cow’s genome. A biotechnology-produced veterinary drug is not the same thing as a genetically modified animal or food.

Sources
Link to this answer
Does bST/rbST end up in milk?Human Safety

Milk naturally contains bovine proteins and hormones. The consumer-safety question is biological activity and dietary exposure — not simply whether a molecule can be detected.

FDA’s safety rationale emphasizes that bST is a large dietary protein degraded by digestive enzymes and that bovine somatotropin does not have biological activity in humans. That is why this hub makes no absolute “zero molecule” claim: the relevant question is biological activity, not detectability.

Sources
Link to this answer
Can bovine somatotropin act as a growth hormone in humans?Human Safety

FDA states that bovine somatotropin does not promote biological activity in the human body.

FDA notes two relevant points: dietary bST is degraded in the gastrointestinal tract, and somatotropins from lower mammalian species do not have growth-promoting activity in humans.

Sources
Link to this answer
Does milk from rbST-treated cows cause cancer?Human Safety

Regulatory reviews have not established that consuming milk from rbST-treated cows increases cancer risk.

Cancer concerns generally focus on IGF-1 rather than direct activity of bST in humans. WHO/JECFA reaffirmed its human food-safety evaluation in 2013. The American Cancer Society notes that evidence of human harm is not conclusive and does not take a formal position on rBGH use. For that reason this answer does not claim that “cancer is impossible.”

Sources
Link to this answer
What about IGF-1 in milk?Human Safety

IGF-1 is naturally present in milk and in the human body. The key question is whether dietary exposure from milk meaningfully changes human biological exposure.

JECFA’s 2013 reassessment reaffirmed its previous food-safety conclusion for rbST. Reviews also note that orally consumed IGF-1 has little or no biological activity and that milk IGF-1 concentrations remain within ranges influenced by normal factors such as farm, parity and stage of lactation.

Sources
Link to this answer
Can rbST milk cause early puberty or abnormal growth in children?Human Safety

Current regulatory evaluations have not identified rbST use in dairy cows as a dietary growth-hormone hazard to children.

This claim is often inferred from the words “growth hormone.” That inference skips species specificity, digestion and exposure. Direct long-term child-specific trials are not the basis of approval, so this answer cites the regulatory food-safety evaluations and goes no further than they do.

Sources
Link to this answer
Is rbST milk linked to diabetes?Human Safety

A published human-health review reported no evidence that rbST use increased type 1 or type 2 diabetes risk in people consuming dairy products.

This finding comes from a published human-health review rather than from FDA’s approval decision, so it carries less weight than the regulatory conclusions above. The original review is listed in this answer’s sources.

Sources
Link to this answer
Does pasteurization make rbST milk safe?Human Safety

The FDA safety conclusion does not depend on pasteurization destroying rbST.

The stronger explanation is that bST is a protein degraded during human digestion and is not biologically active as a growth hormone in humans. That is why this hub does not present pasteurization as the central safety mechanism.

Sources
Link to this answer
Is milk from rbST-treated cows nutritionally different?Milk & Quality

Reviews report no meaningful effect of rbST use on the macro- and microcomposition of milk.

Milk yield and milk composition are different questions. St-Pierre’s meta-analysis found increased milk, fat and protein yields without changing the concentration of milk components; a human-health review similarly reported no effect on micro- or macrocomposition.

Sources
Link to this answer
Does rbST change the taste or quality of milk?Milk & Quality

Evidence reviews focus on composition and safety and do not support a general claim that rbST creates a nutritionally superior or inferior milk.

Taste is influenced by many variables outside rbST use. What the evidence supports is that milk composition is not materially changed by rbST use; it does not promise that every sensory attribute is identical under every farm and processing condition.

Sources
Link to this answer
Does rbST mean more antibiotics in the milk supply?Milk & Antibiotics

FDA’s post-approval review found no product-related increase in violative antibiotic residues in the U.S. milk supply.

FDA acknowledges that sometribove can increase clinical mastitis risk and that mastitis is commonly treated with antibiotics. But tanker testing, withdrawal requirements and residue monitoring are separate safeguards. FDA’s 2013 review found no product-related increase in residues above allowed limits after commercialization.

Sources
Link to this answer
Does rbST contribute to antibiotic resistance?Milk & Antibiotics

The concern is indirect: more mastitis can mean more antibiotic treatment. The extent to which this translates into human antibiotic-resistance risk is uncertain.

The American Cancer Society notes the concern and uncertainty. FDA’s U.S. residue-monitoring analysis addresses illegal residues in milk, which is not the same question as population-level antimicrobial resistance. They are different questions.

Sources
Link to this answer
Is there a milk withdrawal period after Posilac?Milk & Antibiotics

FDA states that Posilac has a zero-day withdrawal for milk and meat.

That means no withdrawal time is required for Posilac itself when used according to the approved label. This does not override withdrawal periods for other veterinary drugs, such as an antibiotic used to treat a separate condition.

Sources
Link to this answer
Does “rBST-free” mean the milk is safer?Milk & Quality

An rBST-free claim describes a production practice; it should not be presented as proof that the milk is safer or more nutritious.

Consumer research shows that process labels can influence perceived value and can stigmatize conventional milk. Consumer choice deserves respect; absence labeling and comparative safety are nonetheless different questions.

Sources
Link to this answer
Does rbST increase mastitis?Animal Health

Some studies and FDA’s original evaluation found an increased risk of clinical mastitis; later meta-analyses using the U.S.-approved formulation and label conditions reported different results.

FDA says increased side effects were observed but were manageable under normal U.S. farming conditions and are communicated in labeling. A 2003 Canadian meta-analysis estimated higher clinical mastitis risk; a 2014 meta-analysis of the U.S. rbST-Zn formulation found no increase in odds of clinical mastitis.

Sources
Link to this answer
Does rbST increase lameness?Animal Health

Evidence differs by dataset and era. An older Canadian meta-analysis found increased clinical lameness; a later meta-analysis of labeled rbST-Zn use did not find increased clinical lameness or lameness lesions.

The studies differ in formulation, inclusion criteria, management context and treatment conditions, which is why a simple yes-or-no answer would misrepresent the evidence.

Sources
Link to this answer
Does rbST affect a treated cow’s fertility?Animal Health

Reproductive outcomes vary across analyses; some older pooled data reported more failure to conceive in treated cows, while a later U.S.-formulation meta-analysis found mixed pregnancy effects.

The 2003 Canadian analysis reported higher risk of non-pregnancy. The 2014 meta-analysis reported a higher first-two-cycle pregnancy proportion but a lower overall trial pregnancy proportion in treated cows, with no effect on days to pregnancy or inseminations per pregnancy.

Sources
Link to this answer
Does rbST shorten a cow’s productive life?Animal Health

Older analyses raised concerns about culling in some groups; later formulation-specific analyses and FDA monitoring should be considered alongside those findings.

The 2003 Canadian review found limited evidence suggesting increased culling, particularly in multiparous cows. FDA, meanwhile, cites post-approval monitoring in more than 1,000 cows and continues surveillance. Neither “no impact ever” nor “rbST shortens every cow’s life” is supported by that evidence.

Sources
Link to this answer
Does rbST 'push cows beyond their natural limits'?Animal Health

That phrase is rhetorical, not a scientific endpoint. The measurable questions are feed intake, body condition, health events, reproduction, lameness and welfare.

Higher milk output does change a cow’s nutrient demand, which is why label-directed use in healthy cows, appropriate nutrition and herd management are essential context.

Sources
Link to this answer
Is Posilac intended for every dairy cow?Animal Health

No. FDA describes Posilac as approved for use in healthy lactating dairy cows according to the product label.

Efficacy and safety findings are tied to labeled use, herd selection, nutrition, veterinary oversight and farm management; the product is not a stand-alone solution.

Sources
Link to this answer
If rbST is safe for people, why is it prohibited in the European Union?Regulatory Context

The EU’s 1999 decision focused on animal-health and animal-welfare concerns, not a finding that milk from treated cows was unsafe for human consumption.

The EU decision cites clinical mastitis, foot and leg disorders, reproduction and injection-site reactions and then prohibits marketing and administration of BST to dairy cows in the EU. It reflects a different regulatory policy framework.

Sources
Link to this answer
Why is rbST not approved in Canada?Regulatory Context

Health Canada states that its review did not identify a human-health risk, but animal-health concerns prevented approval for sale.

This is a useful example of why “not approved” is not automatically synonymous with “unsafe for consumers.” Different jurisdictions can weigh animal-health, welfare and policy considerations differently.

Sources
Link to this answer
Do regulators disagree about rbST?Regulatory Context

They have reached different market-use decisions, while several major reviews agree that the principal controversies include animal health and welfare rather than a demonstrated dietary hazard to humans.

FDA approved Posilac in the U.S.; WHO/JECFA reaffirmed food-safety conclusions; Canada did not approve it because of animal-health concerns; the EU prohibits its administration to dairy cows on animal-health and welfare grounds.

Sources
Link to this answer
Was rbST only studied in the 1990s?Regulatory Context

No. FDA cites post-approval monitoring and continued adverse-event surveillance, and JECFA conducted a literature-based reassessment in 2013.

Original approval data, post-approval surveillance and later scientific reviews are three different things; the 1993 approval did not end scientific review.

Sources
Link to this answer
Does rbST actually increase milk production?Productivity

Yes. Multiple controlled studies and meta-analyses report increased milk yield when rbST is used under studied conditions.

A 2014 meta-analysis of sometribove zinc suspension found mean milk yield increased by about 4.0 kg/day across included studies. A full-lactation trial in a Brazilian commercial herd found cows on the Posilac formulation produced 4.49 kg/day (first lactation) and 5.66 kg/day (older cows) more than untreated controls over a 238-day treatment period. Each figure applies only to the study population and conditions it was measured in.

Sources
Link to this answer
Can rbST reduce the environmental footprint of milk?Sustainability

Modeling studies suggest rbST can reduce resource use and emissions per unit of milk by improving productivity, but these are system-model results rather than universal farm-level guarantees.

Capper and colleagues modeled lower feed, water, land, nutrient excretion, greenhouse-gas emissions and fossil-fuel use for equivalent milk output under rbST scenarios. The study’s authors included people with Monsanto relationships, and the result is modeled evidence rather than a farm-level measurement.

Sources
Link to this answer
Does higher milk yield really mean fewer cows are needed for the same amount of milk?Sustainability

Mathematically, higher output per cow can reduce the number of cows required for a fixed milk target, but the actual system outcome depends on herd management and market behavior.

This is a useful efficiency concept, not a promise that every farm will immediately reduce herd size. Sustainability figures of this kind are expressed per unit of milk and depend on the modeling assumptions behind them.

Sources
Link to this answer
Who has evaluated the food safety of rbST?Regulatory Context

FDA and the Joint FAO/WHO Expert Committee on Food Additives (JECFA) are among the major authorities that have evaluated rbST food safety.

WHO/JECFA’s database records an ADI and MRL as “not specified” and says the committee reaffirmed its earlier conclusion after a systematic literature review in 2013. “Not specified” means the committee saw no need to set a numerical limit on the available data; it does not mean “no rules needed.”

Sources
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About the studies

Where our figures come from.

The figures on this site come from specific studies. Here is what those studies involved, so you can judge how far the results may apply to your farm.

9.90 lb more milk per cow per day (first-lactation cows), 12.47 lb (older cows), and 2,755 lb more per cow over the 238-day treatment period

De Morais et al. 2017, Journal of Dairy Science 100(7):5945-5956

  • One large commercial Holstein herd in Brazil, studied by researchers from the Federal University of São Carlos.
  • 240 cows (35% first-lactation, 65% older cows), compared with untreated cows in the same herd over a full lactation.
  • Only healthy cows entered the study: examined by a veterinarian, producing at least 55 lb (25 kg) of milk a day, in good body condition, with low somatic cell counts and no history of clinical mastitis.
  • 500 mg every 14 days as directed on the label, for 17 cycles (238 days, from about day 63 to day 300 of lactation).
  • The 2,755 lb figure is our calculation from the study's published averages (5.25 kg, about 11.6 lb, a day across the herd, over 238 days). It is not printed in the paper.

7.3% less methane, 8.1% less total feed, 6.8% less manure and 11.8% less maintenance energy per unit of milk, and the one-million-cow comparison (about 400,000 cars or 300 million trees)

Capper et al. 2008, Proceedings of the National Academy of Sciences 105(28):9668-9673

  • A computer model of U.S. dairy production, not a measurement on farms.
  • It compares producing the same amount of milk with and without rbST, for a cow giving 28.9 kg (about 64 lb) of milk a day and responding with 4.5 kg (about 10 lb) more.
  • Some of the authors worked for or consulted for Monsanto, then the maker of Posilac®.

21% of the animals, 35% of the water, 10% of the land and 37% of the carbon footprint

Capper et al. 2009, Journal of Animal Science 87:2160-2167

  • Compares the whole U.S. dairy industry in 2007 with 1944, for the same amount of milk.
  • It describes six decades of gains in productivity overall. It is not a measurement of Posilac®.

These results come from the conditions described above. Farms whose herds, nutrition, management or climate differ, or where conditions are less well controlled, may see different results.

Check us

Don't take our word for it.

A science hub earns trust when readers can leave the page and verify the evidence for themselves.

FDAFDA — Bovine Somatotropin (bST)Open source ↗FDA ABXFDA — bST, Mastitis and Antibiotic Residues (2013)Open source ↗WHOWHO/JECFA — Bovine Somatotropin evaluationOpen source ↗JECFA 2013JECFA 78th report (WHO TRS 988) — 2013 reassessmentOpen source ↗EUEuropean Union — Council Decision 1999/879/ECOpen source ↗CANHealth Canada — Hormonal Growth Promoters FAQOpen source ↗ACSAmerican Cancer Society — rBGH/rBSTOpen source ↗COLLIERCollier et al. 2014 — Human health reviewOpen source ↗2025 REVIEWCollier, Bauman & Baumgard 2025 — Somatotropin and lactation biologyOpen source ↗STPSt-Pierre et al. 2014 — Meta-analysisOpen source ↗DE MORAISDe Morais et al. 2017 — Full-lactation trial, BrazilOpen source ↗BAUMANBauman et al. 1999 — Responses in Northeast U.S. herdsOpen source ↗HARTNELLHartnell et al. 1991 — Sometribove production responsesOpen source ↗POLITISPolitis et al. 1990 — Plasmin and the mammary glandOpen source ↗ETHERTONEtherton & Bauman 1998 — Biology of somatotropinOpen source ↗DOHOODohoo et al. 2003 — Animal health meta-analysisOpen source ↗CAPPERCapper et al. 2008 — Environmental impactOpen source ↗LABELKolodinsky 2008 — rBST-free labeling and perceptionOpen source ↗
From science to practice

Understand rbST. Then decide what fits your herd.

Read the approved product information, talk with your herd-health professionals and use Posilac® only according to its approved label.

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