are inserted into the vagina or anus prior to sexual penetration. These condoms are made of either polyurethane or nitrile. If external and internal condoms are used at the same time, they may break due to friction between the materials during sexual activity. A dental dam (originally used in dentistry) is a sheet of latex used for protection when engaging in oral sex. It is typically used as a barrier between the mouth and the vulva during cunnilingus or between the mouth and the anus during anal–oral sex. Medical gloves made out of latex, vinyl, nitrile, or polyurethane may be used as a makeshift dental dam during oral sex, or can cover hands, fingers, or other body parts during penetration or sexual stimulation, such as masturbation. Condoms, dental dams, and gloves can also be used to cover sex toys such as dildos during sexual stimulation or penetration. If a sex toy is to be used in more than one orifice or partner, a condom/dental dam/glove can be used over it and changed when the toy is moved. Oil-based lubrication can break down the structure of latex condoms, dental dams or gloves, reducing their effectiveness for STI protection. While use of external condoms can reduce STI risks during sexual activity, they are not 100% effective. One study has suggested condoms might reduce HIV transmission by 85% to 95%; effectiveness beyond 95% was deemed unlikely because of slippage, breakage, and incorrect use. It also said, "In practice, inconsistent use may reduce the overall effectiveness of condoms to as low as 60–70%".p. 40. Pre-exposure prophylaxis (PrEP) Pre-exposure prophylaxis (often abbreviated as PrEP) is the use of prescription drugs by those who do not have HIV to prevent HIV infection. PrEP drugs are taken prior to HIV exposure to prevent the transmission of the virus, usually between sexual partners. PrEP drugs do not prevent other STI infections or pregnancy. As of 2018, the most-widely approved form of PrEP combines two drugs (tenofovir and emtricitabine) in one pill. That drug combination is sold under the brand name Truvada by Gilead Sciences. It is also sold in generic formulations worldwide. Other drugs and modalities are being studied for use as PrEP. Different countries have approved different protocols for using the tenofovir/emtricitabine-combination drug as PrEP. That two-drug combination has been shown to prevent HIV infection in different populations when taken daily, intermittently, and on demand. Numerous studies