The HSUS studies porcine zona pellucida (PZP) under the auspices of an Investigational New Animal Drug (INAD) exemption from the federal Food and Drug Administration (FDA). The INAD is the FDA's mechanism for authorizing and guiding research directed at moving new drugs through the approval process. Under FDA rules, a drug or vaccine being studied under an INAD cannot be described by its sponsor as "safe" for a specific use. However, the FDA does review all research projects for scientific validity, relevance to the drug approval process, target animal safety, and human food safety.

The total volume of a PZP injection is 1 cc (approximately 1/5 teaspoon). In general, there are two components to the PZP vaccine. The first is the PZP itself, which is a family of pig proteins extracted from pig ovaries by simple physical and chemical processes, dissolved in a saline solution. There is absolutely no evidence that PZP in the form we use it is physiologically or immunologically active when eaten—which is why we go to the trouble of injecting it rather than administering it using an easier method. The second component of the vaccine is an adjuvant, a substance that boosts the action of the immune system.

The initial research with PZP was performed using Freund's Complete Adjuvant (FCA) for the initial shot and Freund's Incomplete Adjuvant (FIA) for subsequent shots. Both adjuvants consist principally of mineral oil; FCA also contains killed fragments of Mycobacterium tuberculin, the bacteria that causes tuberculosis. We treated hundreds of deer and horses with injections of PZP and FCA and saw only minor local reactions (swellings and, rarely, draining abscesses). However, because FCA can cause false positive readings in tuberculosis tests in deer (FCA does not cause tuberculosis), has a history of provoking severe inflammatory reactions in laboratory animals, and perhaps for other reasons as well, the FDA rejected the use of FCA in a commercial product. Consequently, The HSUS no longer uses FCA in deer immunocontraception studies.

A variety of commercial and experimental adjuvants are under consideration by The HSUS and other investigators, and suitable alternatives with better safety records than FCA have been found. Over the last quarter-century, thousands of animals belonging to dozens of species have been treated with the PZP vaccine. Side effects of PZP that have been reported so far include the injection site reactions described above, ovarian abnormalities in dogs, an extended breeding season in deer, and ovarian and bone marrow abnormalities in deer. No one has yet documented that the extended breeding seasons or the observed abnormalities are harmful, but these areas are under active investigation.