Punicalagin is an ellagitannin, a type of phenolic compound. It is found in forms alpha and beta in pomegranates (Punica granatum), in Terminalia catappa and Terminalia myriocarpa, and in Combretum molle, the velvet bushwillow, a plant species found in South Africa. These three genera are all Myrtales and the last two are both Combretaceae.
Punicalagins are water-soluble and hydrolyze into smaller phenolic compounds, such as ellagic acid. There were no toxic effects in rats on a 6% diet of punicalagins for 37 days. In laboratory research, punicalagins had carbonic anhydrase inhibitor activity.

Punicalagins are large molecules (ellagitannins) found in Pomegranates and Pomegranate Juice. They themselves are very potent anti-oxidants, and they can be metabolized into other compounds (ellagic acid, urolithins) that themselves have anti-oxidant capabilities (although lesser). This large anti-oxidant value of punicalagins, about thrice that of red wine and green tea, is what brought Pomegranates to fame. That being said, the overall amount of punicalagins that get into the bloodstream after oral ingestion is quite small (6% is the highest noted and was in rats, sometimes human studies don't even notice punicalagins in the blood). The ellagic acid and urolithins still confer a lot of health benefits, but their anti-oxidant capabilities are on par with green tea and red wine if not a bit less potent. There is a very high variation between individuals, and like daidzein (one of the soy isoflavones) it may be genetically dependent on what intestinal microflora one possesses.
Like green tea, pomegranate is also rich in polyphenolic flavonoids. The phenol punicalagin is responsible for more than 50% of the juice's potent antioxidant activity. It appears that pomegranate juice has higher levels of punicalagin because the rind is used in its production. In fact, there is actually more antioxidant activity from the tannins in the juice than there is in the fruit itself. A 2-year, single-center clinical trial was completed for 48 men with prostate cancer and a rising PSA level after either surgery or radiotherapy for the treatment of prostate cancer.