PrEP is a medication regimen taken by HIV-negative people to reduce their risk of acquiring Human Immunodeficiency Virus (HIV). This prevention tool uses antiretroviral drugs, typically a combination of two medicines, to stop the virus from establishing a permanent infection. When one partner uses PrEP, a common question arises about the safety level for the other, unmedicated partner. Understanding PrEP’s specific function—who it protects and how—is necessary to assess the safety dynamics within the relationship.
How PrEP Protects the Person Taking It
PrEP creates a chemical barrier against the HIV virus inside the body of the person taking the medication. The antiretroviral compounds are absorbed into the bloodstream and permeate tissues where HIV first attempts to establish itself, such as the mucosal lining of the rectum or the vagina. These drugs build up concentrations in the immune cells (CD4+ T-cells) that HIV targets for infection. If HIV enters the body, the medication actively intercepts the virus’s life cycle by inhibiting reverse transcriptase. By blocking this core viral enzyme, the virus cannot convert its genetic material, replicate, or spread, stopping the infection before it takes hold.
Direct Safety for the Partner Not Taking PrEP
The partner not taking PrEP receives no direct pharmacological protection from the medication. PrEP drugs do not transfer through sexual contact into the unmedicated partner’s bloodstream or tissues in sufficient quantity to provide protection. Safety for the unmedicated partner is a form of secondary protection that relies entirely on the PrEP user remaining HIV-negative.
This differs from Treatment as Prevention (TasP), where a person with HIV suppresses their viral load to an undetectable level. When a person with HIV achieves an undetectable viral load (U=U), they cannot sexually transmit the virus. While PrEP protects the HIV-negative person and TasP eliminates transmission risk from the HIV-positive person, both strategies result in a near-zero risk of transmission.
Why Consistent Adherence is Essential for Safety
The high level of protection PrEP provides, and the secondary safety for the partner, depends entirely on consistent adherence to the prescribed dosing schedule. PrEP is up to 99% effective at preventing sexual transmission of HIV when taken daily as directed. Missing doses allows the medication concentration in key tissues to drop below the necessary protective threshold.
Drug levels decline quickly, meaning a missed dose compromises the safety of both partners. For maximum protection, especially for receptive vaginal sex, it can take approximately 21 days of daily use to reach optimal drug levels in the tissue. Inconsistent adherence increases the risk of HIV infection for the medicated partner, which reintroduces the risk of transmission to the unmedicated partner. Regular monitoring, including quarterly HIV and STI testing, is a necessary part of the PrEP regimen to confirm the medicated partner remains HIV-negative.
PrEP and Protection from Other Sexually Transmitted Infections
PrEP only prevents the acquisition of HIV. The medication offers no protection against other common sexually transmitted infections (STIs), whether bacterial or viral. This includes infections such as gonorrhea, chlamydia, syphilis, human papillomavirus (HPV), and herpes.
Since PrEP does not protect against these pathogens, barrier methods like condoms remain a necessary tool for comprehensive sexual health. Using condoms alongside an effective PrEP regimen provides the most complete protection package, reducing the risk of HIV acquisition while also preventing the transmission of other STIs. Regular screening for STIs is still recommended for both partners.