Withania somnifera is commonly called Ashwagandha, winter cherry, poison gooseberry, and Indian ginseng. It is a plant substance in the nightshade family. It is normally used as an herbal supplement in Ayurvedic medicine treatments.
One research article (1) looked at Withania somnifera and the treatment of prostate cancer cells. It noted that cell cycle deregulation is highly linked to the pathogenesis of prostate cancer. The study was an in vitro study, meaning that the prostate cancer cells were mixed with an extract of Withania somnifera in a culture medium.
While it found that Withania somnifera was able to stop the growth of prostate cancer cells, this was an in vitro study. More research needs to be done on patients with prostate cancer to see if prostate cancer in humans can be treated with the substance.
Source: 1.Rov RV, et al. Withaferin A, a steroidal lactone from Withania somnifera, induces mitotic catastrophe and growth arrest in prostate cancer cells. J Nat Prod. 2013 Oct 25;76(10):1909-15.