I will need to defer to our experts on this one, but for now here is some info I dug up on Sour Sop:
Soursop (scientific name - Annona muricata), also called Graviola (Portuguese), is a fruit that generally grows in the rain forests of Africa, South America, and Southeast Asia. It has other names like thorny custard apple, cherimoya and brazilian pawpaw. In various languages, this fruit is referred as: guanabana (Spanish), corossol (French), aluguntugui (Ghana), sorsaka (Papiamento), adunu (Acholi), guyabano, guanavana, durian benggala, nangka blanda, sirsak, toge-banreisi, nangka londa and zuurzak. In India, it is less known as shul-ram-fal and hanuman fal, and as mullaatha in Malayalam. The bark, leaves, root and fruits of this tree are used for traditional remedies in many countries. Graviola extracts are used for treating infections of viruses or parasites, rheumatism, arthritis, diarrhea, dysentery, depression and sickness.
The idea that Soursop can fight cancer effectively started after a research at Purdue University's School of Pharmacy and Pharmaceutical Sciences. The research concluded that the active components of the tree are a unique phytochemical substances known as annonaceous acetogenins, which MAY have chemotherapeutic potential, especially with regard to multi drug-resistant cancer cells. But these tests were only confined to test tubes, no large scale clinical trials were conducted on humans to determine the safety and efficacy of Graviola for treating cancer. Therefore, there is no significant evidence to show that Soursop works as a cure for cancer. Even Wikipedia says the same
There is evidence indicating that the fruit's extracts selectively inhibit the growth of human breast cancer cells by downregulating expression of epidermal growth factor receptor (EGFR) in vitro and in a mouse model, but the effect has not been studied in humans.
Moreover, studies show that use of Soursop can have certain adverse effects in some people, especially movement disorders and nerve damage that is similar to Parkinson's disease, which is due to the very high concentration of annonacin. Graviola also has few other side effects like lowering the blood pressure, so it should not be taken by people with low blood pressure or heart complications. The antimicrobial properties of Soursop can also kill beneficial bacteria on the skin, in the vagina and gut, which can lead to infections in long term use.
Therefore, the message saying that Graviola, i.e Soursop is 10,000 times more effective cancer killer than chemo is a hoax. However, you can find thousands of websites online selling it as a miracle fruit. We advise people not to believe them blindly, but consult a doctor or oncologist before using it.