How Is Premarin Made From Pregnant Mare’s Urine?

Premarin is manufactured by collecting urine from pregnant mares, extracting the naturally occurring estrogens from it, and processing those hormones into tablets, creams, and other pharmaceutical forms. The drug’s name is literally derived from its source: pregnant mare urine. What makes the process unusual in modern pharmacy is that the active ingredients are a complex cocktail of at least ten distinct estrogens, several of which are unique to horses and cannot be synthesized as a group from scratch in a lab.

Why Pregnant Mares Produce So Much Estrogen

Scientists recognized in the late 1920s that the urine of pregnant women and pregnant mares contained high concentrations of estrogens, but it took until the 1940s before anyone demonstrated that mare urine was a practical commercial source.¹ The reason mare urine won out over human urine is partly one of volume and concentration: a pregnant mare produces vastly more urine per day than a pregnant woman, and her estrogen levels are extraordinarily high during pregnancy. In mares, the pattern of estrogen production closely tracks the growth and later regression of the fetal gonads, meaning estrogen output rises dramatically during certain months of gestation and then tapers off.

Horses also produce a family of estrogens that humans do not. The mare’s body uses an alternative biochemical pathway that retains an extra double bond during hormone synthesis, yielding what researchers call “ring B unsaturated” estrogens, the most well-known being equilin.¹ These horse-specific estrogens are the defining feature of Premarin and the main reason the drug has never been fully replicated by synthetic chemistry alone.

From Farm to Pharmacy

The original drug was developed by the Canadian pharmaceutical firm Ayerst, McKenna and Harrison, which later became part of Wyeth (now owned by Pfizer).² The basic production chain has remained conceptually the same since the 1940s, even as the scale has changed enormously. Pregnant mares on specialized farms are fitted with urine-collection devices during the later months of gestation, when estrogen output is highest. The collected urine is shipped to processing facilities where the estrogens are isolated, purified, and formulated into pharmaceutical products.

In Canada and North Dakota, the traditional setup involves housing pregnant mares in tie stalls, where a rubber cup or bag is attached to the mare’s hindquarters to catch urine. Farms in this system have historically kept large herds; one observational study of Canadian operations found an average of about 115 production mares per farm.³ The mares typically spend the roughly six-month collection season in these stalls, then are turned out to pasture after foaling. The annual cycle of breeding, gestation, collection, foaling, and re-breeding keeps a continuous pipeline of estrogen-rich urine flowing to the manufacturer.

How the Urine Is Collected and Why It Sparks Debate

The tie-stall system has been the lightning rod for most of the controversy surrounding Premarin. Mares housed in stalls for months at a time have limited ability to move, socialize, or engage in normal equine behaviors like grazing and rolling. Animal welfare advocates have long argued that this confinement causes stress and discomfort. Supporters of the industry counter that the mares receive veterinary care, regular feeding, and that many draft-breed horses used in production are temperamentally suited to indoor housing.

Researchers in Australia developed an alternative collection system that allows mares to be loose-housed in indoor stables or outdoors in paddocks, rather than tethered in stalls.⁴ This approach uses a different kind of wearable collection device that the mares carry while moving freely. The study evaluating this system measured both behavioral and physiological indicators of welfare, aiming to determine whether the novel device caused distress. The existence of such research reflects a broader push within the industry to find collection methods that are both commercially viable and less restrictive for the animals.

A related welfare concern involves the foals. Each year’s crop of foals is a byproduct of the production cycle. Some are kept for future breeding stock, some are adopted or sold as riding horses, and some have historically ended up at auction, where the risk of slaughter exists. This has been another focal point for advocacy groups, though tracking exact numbers is difficult because the foals enter a secondary market with limited transparency.

What Is Actually in the Pill

Premarin is not a single drug molecule. It contains at least ten biologically active estrogens, all present as sulfate esters, a chemical form that makes them water-soluble and orally absorbable.⁵ Some of these are familiar human estrogens: estrone, 17β-estradiol, and 17α-estradiol. But the majority are horse-specific compounds not found naturally in the human body:

Equilin: the signature equine estrogen, produced through the mare’s unique biosynthetic pathway
Equilenin: a related ring B unsaturated estrogen with potent biological activity
Dihydroequilin and dihydroequilenin: present in both 17α and 17β forms
Δ8-estrone: another ring B unsaturated variant

All ten of these estrogens bind to human estrogen receptors and produce measurable estrogenic effects in the body. Some individual components, including equilin sulfate, Δ8-estrone sulfate, 17β-dihydroequilin sulfate, and estrone sulfate, have particularly potent estrogenic activity on their own.⁵ This complexity is part of what makes Premarin difficult to replicate: it is not one hormone but a blend whose proportions reflect the natural output of a pregnant mare.

How Horse Estrogens Interact with Human Receptors

One of the more interesting pharmacological questions about Premarin is why horse estrogens work in the human body at all, given that several of them are molecules humans never produce. The answer lies in how estrogen receptors work. Humans have two main types, known as ERα and ERβ, and the equine estrogens bind to both, though with different affinities and functional consequences than human 17β-estradiol.

Research on these interactions found that most of the ring B unsaturated estrogens from horses bind to ERα and ERβ with roughly two to eight times lower affinity than 17β-estradiol. However, some of these horse-specific estrogens actually show a preference for ERβ, binding with two to four times greater affinity for ERβ than ERα.⁶ This matters because ERα and ERβ are distributed differently across tissues. ERα predominates in the uterus and breast, while ERβ is more common in the brain, bone, and cardiovascular system. The equine estrogens’ preference for ERβ means they may exert effects in certain tissues that pure 17β-estradiol would not, for better or worse.

When both receptor subtypes were present, some equine estrogens showed enhanced activity: 17β-dihydroequilin’s activity roughly doubled, and 17β-dihydroequilenin’s rose by about 60% compared to conditions with a single receptor type.⁶ Equilenin turned out to be the most active equine estrogen via ERβ. These findings suggest that Premarin’s clinical effects are not simply “estrogen replacement” in a generic sense but a particular pharmacological profile shaped by the mix of horse and human estrogens acting through different receptor pathways. This receptor selectivity has led some researchers to propose that individual equine estrogens could be isolated and used to target specific tissues, though that work remains mostly preclinical.

Neuroprotective Effects of Specific Components

One line of research that has kept interest in Premarin’s complex formulation alive is the finding that several of its individual estrogens protect brain cells from damage. In laboratory studies using cultured neurons, eight of the estrogens found in Premarin significantly reduced neuronal membrane damage caused by glutamate excitotoxicity, a process implicated in stroke and neurodegenerative disease.⁷ Of those, 17β-estradiol and Δ8,9-dehydroestrone stood out for their ability to protect neurons against the toxic effects of beta-amyloid, the protein fragment associated with Alzheimer’s disease.

Perhaps more striking, when two of three neuroprotective estrogens (17β-estradiol, equilin, and Δ8,9-dehydroestrone) were combined, the protective effect was greater than any single estrogen achieved alone. This suggests that part of Premarin’s pharmacological story is about synergy between components, not just the action of any one hormone. The research used computer modeling to link the structural features of these estrogens to their neuroprotective potency, finding that the way each molecule interacts with estrogen receptors predicted how well it protected neurons. These are cell-culture results, not clinical trials in patients, so they should be interpreted cautiously. But they illustrate why the drug’s complexity is both a feature and a challenge: replicating it means replicating not just individual molecules but their combined effects.

Can Synthetic Alternatives Replace Premarin?

The animal welfare concerns and the sheer biological complexity of Premarin have driven efforts to create plant-based synthetic alternatives. One such product is a formulation of nine synthetic conjugated estrogens chemically derived from soybean and yam extracts. Researchers tested this synthetic version head-to-head against Premarin in cultured hippocampal neurons and found that its neuroprotective effects against beta-amyloid, hydrogen peroxide, and glutamate-induced toxicity were comparable to those of the horse-derived formulation.⁸

Synthetic conjugated estrogen products do exist on the market and have been prescribed as alternatives, but they are not identical to Premarin. The horse-derived formulation contains a broader and more variable mix of estrogens, and the proportions naturally fluctuate to some degree between batches. Whether these differences matter clinically has been debated for decades. In one small study comparing Premarin plus calcium against a water-soluble combined estrogen and calcium preparation, both treatments reduced markers of bone turnover and improved menopausal symptoms like hot flashes and vaginal dryness, with no significant difference between the two groups.⁹ Results like these suggest that for managing common menopausal symptoms, the practical differences between conjugated equine estrogens and alternatives may be modest.

Another common alternative is 17β-estradiol delivered through a skin patch. Early comparative studies showed that transdermal estradiol could control postmenopausal symptoms in patients previously managed on conjugated equine estrogens at standard doses.¹⁰ Transdermal delivery bypasses the liver on first pass, which changes the drug’s effects on clotting factors and blood lipids compared to oral estrogens. A study measuring hemostatic parameters found that oral conjugated equine estrogens significantly decreased fibrinogen, tissue plasminogen activator, and plasminogen activator inhibitor-1 levels, while transdermal estradiol had a minimal effect on these clotting-related proteins.¹¹ Whether those differences translate into meaningful changes in blood clot risk is a separate and harder question, but it is one reason clinicians sometimes choose one delivery route over another.

How the Women’s Health Initiative Changed Everything

No discussion of Premarin’s place in medicine is complete without mentioning the Women’s Health Initiative (WHI), the massive clinical trial that upended hormone therapy prescribing in 2002. The WHI’s estrogen-plus-progestin arm, which used Premarin combined with medroxyprogesterone acetate (sold as Prempro), was stopped early after researchers found increased risks of breast cancer, heart disease, stroke, and blood clots. The fallout was swift and dramatic: within nine months, prescriptions for hormone therapy dropped by about a third, and promotional spending by manufacturers fell sharply. Direct-to-consumer advertising for hormone therapy dropped to zero. The greatest promotional decline was for standard-dose Prempro itself, the specific product implicated by the trial, which saw promotional spending fall by roughly 60%.¹²

The WHI results hit Premarin-based products harder than any animal welfare campaign ever had. Prescriptions that had been climbing for decades reversed course almost overnight. The nuances took longer to emerge: later analyses showed that the risks were concentrated in older women who started therapy well past menopause, and that estrogen-alone therapy (without progestin) had a somewhat different risk profile. But the immediate effect was a cultural shift in how both doctors and patients viewed hormone replacement. Premarin went from being one of the most prescribed drugs in the United States to a treatment viewed with much more caution.

The Draft Horse Connection

An aspect of Premarin production that surprises many people is the types of horses involved. The industry has historically relied heavily on draft breeds and draft crosses, large-bodied horses that produce more urine per day and tolerate confinement in stalls better than lighter breeds. Research comparing draft mares to lighter mares in the tie-stall system found differences in how the two types spent their time, with draft mares generally showing behavioral patterns suggesting better adaptation to the stall environment.³ Draft breeds like Percherons and Belgians, originally bred for farm work, had already declined in numbers as mechanized agriculture replaced horse-drawn equipment. The PMU industry provided a new economic niche for these breeds, which is part of why some breed registries and horse industry groups have been less hostile to Premarin production than the broader animal welfare community.

When prescriptions fell after the WHI, so did the demand for pregnant mare urine, and thousands of mares were removed from production. Many were rehomed through adoption programs, but the sudden surplus created its own welfare challenges. The fate of these horses became a secondary controversy layered on top of the primary one about collection conditions. For people who care about equine welfare, Premarin remains a topic where pharmaceutical science, agricultural economics, and animal ethics intersect in uncomfortably tangled ways.