Pre-Exposure Prophylaxis (PrEP) is a highly effective medication designed for HIV-negative individuals to prevent them from acquiring the virus. It acts as a shield, providing protection before potential exposure. PrEP requires confirmation of HIV-negative status through testing before starting the regimen. When an individual who is already HIV-positive takes this preventative medication, it creates a serious medical complication that can undermine future treatment success.
Prevention Versus Full Treatment
The difference between PrEP and full Antiretroviral Therapy (ART) lies in their purpose and composition. PrEP, typically a single daily pill, contains one or two active antiretroviral compounds, such as a combination of tenofovir and emtricitabine. These drugs are sufficient to block the initial steps of the virus’s replication cycle in an HIV-negative person, preventing the infection from taking hold.
ART is a robust regimen requiring a combination of at least three different drugs from multiple pharmacological classes. This multi-drug approach is necessary to suppress the virus in an HIV-positive person, where the virus is already actively replicating. The goal of ART is to reduce the viral load—the amount of HIV in the bloodstream—to an undetectable level, which protects the person’s health and prevents transmission.
The minimal drug combination in PrEP is intentionally inadequate for treating an established HIV infection. The virus is constantly mutating and replicating, demanding the multi-class regimen of ART to overwhelm it. PrEP is strictly a preventative measure, not a treatment for an existing infection.
Why PrEP Alone Causes Drug Resistance
The most significant medical consequence of taking PrEP while HIV-positive is the rapid development of drug resistance within the virus. The incomplete regimen provides “selective pressure” on the actively replicating HIV population. The one or two drugs in the PrEP pill inhibit the majority of the virus, but they do not fully suppress it.
During replication, HIV naturally makes millions of copies, and small genetic errors, or mutations, occur. Most of these mutations are harmless or create weaker viruses that are easily suppressed by the PrEP drugs. However, a small number of these mutations may, by chance, make the virus resistant to the specific drugs in the PrEP regimen, such as tenofovir or emtricitabine.
Because the PrEP drugs are only partially suppressing the virus, the drug-sensitive strains die off, but the drug-resistant strains survive and multiply unimpeded. This selective pressure allows the resistant version of the virus to quickly become the dominant strain in the body. This acquired resistance means that when the person starts a full ART regimen, two commonly used first-line drugs will already be ineffective. This significantly limits future treatment options available for long-term health management.
Critical Steps After Discovery
If an individual discovers they are HIV-positive while taking PrEP, the most urgent step is to stop taking the PrEP medication and contact a healthcare provider specializing in HIV care. Continuing the partial treatment will only accelerate the drug resistance process. The healthcare team will conduct a comprehensive baseline assessment to determine the extent of the infection and guide the next steps.
This initial assessment includes a viral load test to measure the amount of virus in the blood and a CD4 count to check the health of the immune system. Crucially, the provider must order a genotypic resistance test. This test analyzes the genetic makeup of the individual’s HIV to identify specific mutations that confer resistance to antiretroviral drugs, particularly the components of PrEP. For example, the M184V mutation causes resistance to emtricitabine and lamivudine.
The results of the genotypic test will dictate the composition of the new, full ART regimen. The goal is to bypass the drugs to which the virus has developed resistance and select a combination of three or more fully active drugs from different classes. Treatment should not be delayed while awaiting the full resistance results; a provider will often start a provisional, robust ART regimen immediately and then modify it once the mutation data is available. Prompt transition to a full, effective ART regimen is paramount to suppressing the virus, preserving the immune system, and protecting the individual’s long-term health.