Phase I Safety Trials determine the safety of the product when used by 20 to 50 healthy, low-risk volunteers over several weeks or months. In the case of microbicides, scientists look to see whether the product causes irritation or negatively affects the vagina or rectum's normal environment.

Phase II Expanded Safety Trials test the safety of the product in a larger number of volunteers over a longer period of time. Phase II trials generally take place among volunteers whose risk profile resembles that of the women or men who will be enrolled in large-scale effectiveness trials.

Safety must also be assessed in HIV+ individuals because some HIV+ people will want to use those products and many users will not know their HIV status when they ultimately use a microbicide. Penile safety studies are normally carried out as well to ensure that product is safe and acceptable for the user's male partner.

Phase IIb Trials, also called proof-of-concept trials, are designed to address the difficulty of predicting whether a microbicide will protect against HIV. Larger than a Phase II study but often only half the size of a full-scale Phase III trial, a Phase IIb study enrols enough people to offer a suggestion of whether the microbicide can prevent HIV infection.

Phase IIb studies can be used to eliminate products that clearly don’t work (and therefore aren’t worth testing in a full Phase III trial) or to help identify which of several products makes the most sense to move forward into Phase III.

In some cases, the same participants who take part in Phase II or Phase IIb trials may choose to “roll over” into a Phase III trial.

Participant safety is the primary concern in all of these studies.

Phase III Effectiveness Trials enrol thousands of people in several sites, and measure whether or not the microbicide actually works to prevent HIV and STIs. Some Phase II trials of microbicides can "roll into" Phase III trials as long as the data show good results.