It doesn't seem like anyone has posted on this and since Jon mentioned it in his post, I thought I should include the whole story:
Carraguard, a gel substance made from seaweed, was being investigated as a possible microbicide for protection against HIV. Results of the trial, though, found that those that used the carraguard got HIV at the same rate as those that used the control gel. A number of explanations have arisen, including that the women in the experimental group didn't use the gel as much as they were supposed to. The experimental group did not have any serious complications from the gel, prompting investigation into carraguard as a base into which antivirals could be added in the future.