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  1. Product Safety Information

Bovine Somatotropin (bST)


Bovine somatotropin (bST), also known as bovine growth hormone, is an animal drug approved by FDA to increase milk production in dairy cows. This drug is based on the somatotropin naturally produced in cattle. Somatotropin is a protein hormone produced in the pituitary gland of animals, including humans, and is essential for normal growth, development, and health maintenance.

Early research in the 1930s and 1940s in Russia and England found that milk production in cows could be increased by injecting cattle pituitary extracts, specifically bST. English scientists attempted to increase milk production in cows during World War II with pituitary-derived bST to alleviate food shortages. However, it wasn’t until the 1980s that it became technically possible and economically feasible to produce large commercial quantities of bST by a process using biotechnology. The bST derived by this process is typically called “recombinant” bST or “rbST”. This process is also advantageous for producing a more consistent and purified source of bST.

The FDA approves an animal drug for food-producing animals only after information and/or studies have shown that the food (in this case, milk and meat) from the treated animals is safe for people to eat, and that the drug does not harm treated animals, or the environment. The drug also must be effective, meaning that it works as intended. The labeling for animal drug products provides all instructions for safe and effective use and is approved by FDA. The FDA publishes public documents (Freedom of Information Summaries) on its website that summarize the information that FDA used to determine that the drug is safe for the treated animals, the animal products (edible tissues such as meat and milk) are safe for humans to eat, and that the product is effective.

FDA approved a bST product in 1993 with the brand name “Posilac™” (sometribove zinc suspension) after determining that its use would be safe and effective. Posilac™ is approved for over-the-counter use in dairy cows starting at around 2 months after the cow has a calf until the end of the lactation period. During this time, cows are injected with Posilac™ subcutaneously (under the skin) every 14 days. A cow’s typical lactation period is approximately 10 months long, starting right after she has a calf. Thus, treated dairy cows are typically given Posilac™ for about 8 months of the year. After the 10-month lactation period, the producer stops milking the cow to allow a 2-month period for her mammary gland to rest and regenerate before she has a calf and starts the next lactation period.

A key factor in FDA’s determination that milk and meat from cows treated with Posilac™ is safe for humans to eat is that bST is a large protein. Like most dietary proteins, bST is degraded by digestive enzymes in the gastrointestinal tract and not absorbed intact. The digested proteolytic fragments have no biological activity. Furthermore, even if it was injected, bST does not promote biological activity in the human body because somatotropins from lower mammalian species have no activity in humans. Posilac™ has a zero day withdrawal. This means that the milk and meat from cows is safe for humans to eat at any time after the animal is treated with Posilac™. Numerous international health and food safety organizations and many national regulatory agencies have confirmed the safety of milk and meat from bST-treated cows for human consumption. Additional information on the safety of milk and meat from treated cows is provided in the references below.

FDA evaluated the results of numerous studies before approval to determine if Posilac™ was safe for treated dairy cows. Many of the studies were conducted under typical farm conditions in multiple regions of the U.S. The effects of treatment were evaluated on all aspects of cows’ health, feed intake, ability to have a calf, and the health of their offspring. FDA concluded that Posilac™ was safe to use in healthy dairy cows. Some increased side effects were observed in Posilac™-treated cows, but FDA determined that these effects were manageable under normal U.S. farming conditions. The labeling for the product identifies the potential side effects to treated cows so that dairy farmers can make an informed decision on whether to use Posilac™ in their herd. A post-approval monitoring study in 28 herds around the U.S. and including more than 1000 dairy cows confirmed the safety of the product to treated cows. Additional information on the safety of Posilac™ for dairy cows is provided in the references below.

As with all drugs approved by FDA, the Agency continues to monitor the safety and effectiveness of Posilac™ through post-approval surveillance and review of all reported adverse drug experiences.

For more information about bST, see the following reports and approval documents.


Citizen Petition

Freedom of Information Summaries

Environmental Impact Summaries


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