Final answer:
Nonoxynol-9 is the spermicide that has been found to be less effective at preventing STI's and increases the risk of contracting HPV and HIV with continued use.
Step-by-step explanation:
The spermicide that has been found to be less effective at preventing STIs and increases the risk of contracting HPV and HIV with continued use is nonoxynol-9. Nonoxynol-9 is a widely used spermicide that works by killing or immobilizing sperm. However, studies have shown that it can cause irritation and disrupt the natural balance of bacteria in the vagina, making it easier for STIs like HPV and HIV to be transmitted.