Drug-resistant HIV is treated by building a new combination of medications that the virus hasn’t developed defenses against. The goal is the same as standard HIV treatment: suppress the virus to undetectable levels. Getting there requires resistance testing to identify which drugs still work, then assembling a regimen with at least two fully active medications. For people with extensive resistance, newer drug classes that attack HIV in completely different ways have expanded options significantly in recent years.
How HIV Becomes Drug-Resistant
HIV copies itself billions of times a day, and each copy can contain small errors in its genetic code. Some of these errors, called mutations, change the shape of the proteins that HIV drugs target. When that happens, the drug can no longer latch onto the virus effectively, and those mutated copies survive and multiply while the non-mutated ones are killed off. Over time, the resistant strain becomes dominant.
The most common driver of resistance is inconsistent medication use. Missing doses, taking pills at irregular times, or stopping and restarting treatment all create windows where drug levels in the blood drop low enough for the virus to replicate, but high enough to put selective pressure on it. Drug interactions that lower medication levels in the body can have a similar effect. Less commonly, a person can acquire a resistant strain at the time of initial infection, meaning their virus already carries mutations before they ever start treatment.
Different mutations affect different drug classes. One well-known mutation makes the virus resistant to two widely used medications (lamivudine and emtricitabine) but also weakens the virus’s ability to copy itself efficiently. Another common mutation confers resistance to an older class of drugs without significantly slowing the virus down, making it more likely to persist and spread. These patterns matter because they determine which medications remain useful in a new regimen.
Resistance Testing: Finding What Still Works
Before switching medications, your healthcare team will order resistance testing to map out which drugs the virus can still be affected by. There are two main types, and most people only need the first one.
Genotypic testing reads the virus’s genetic code and looks for known resistance mutations. It’s the standard approach: faster, less expensive, and more sensitive at detecting mixtures of resistant and non-resistant virus. Results typically come back within one to two weeks. This test requires a detectable viral load, generally at least 500 to 1,000 copies per milliliter of blood.
Phenotypic testing takes a different approach. It grows the virus in a lab alongside increasing concentrations of each drug to directly measure how much drug is needed to stop it. This is more like a real-world stress test. Results take two to three weeks and cost more, so phenotypic testing is typically reserved for people whose resistance patterns are complex or hard to interpret from genetic data alone. For most situations, including first or second treatment failures, genotypic testing is preferred.
Your treatment history also plays a major role. Even if a mutation doesn’t show up on a current test, it may still be lurking at low levels if you took a drug in the past and developed resistance to it. Clinicians use both the test results and a detailed medication history to piece together the full picture.
Building a New Regimen
The core principle of treating drug-resistant HIV is straightforward: you need enough fully active drugs in the new combination to overpower the virus. Current guidelines lay out a clear hierarchy for how to do this.
If at least one drug with a high barrier to resistance is still fully active (meaning the virus shows no mutations against it), two fully active drugs can be enough. High-barrier drugs are those that require the virus to accumulate many mutations before resistance develops. If no high-barrier drug is available, the target shifts to at least three fully active medications. When even that isn’t possible, the regimen should include as many active drugs as can be assembled, supplemented by partially active ones that may still contribute some antiviral pressure.
This is why the newer drug classes described below have been so important. They give clinicians additional fully active options for people who have run through multiple previous regimens.
Newer Drug Classes for Extensive Resistance
For people whose virus has developed resistance to multiple standard drug classes, three newer medications work through mechanisms the virus has never encountered, making cross-resistance unlikely.
Capsid Inhibitors
Lenacapavir (brand name Sunlenca) is the first drug in its class. It targets the protein shell, or capsid, that surrounds HIV’s genetic material. What makes it unusual is that it disrupts multiple stages of the virus’s life cycle: it blocks the virus from delivering its DNA into the cell’s nucleus, interferes with the assembly of new virus particles, and causes newly formed capsids to be structurally defective. Because no previous HIV drug works this way, existing resistance mutations don’t affect it.
The dosing schedule is also distinctive. After a short oral loading period, lenacapavir is given as an injection under the skin once every six months. Its half-life of 8 to 12 weeks makes this long interval possible. It was approved in the EU in 2022 specifically for drug-resistant HIV when other options are insufficient, and it’s used in combination with whatever other active drugs can be assembled.
Attachment Inhibitors
Fostemsavir (brand name Rukobia) prevents HIV from attaching to CD4 cells in the first place. It targets a protein on the surface of the virus itself, blocking the very first step of infection. This is a 600 mg tablet taken twice daily. Because it works at a completely different point in the viral life cycle than older drugs, resistance to those drugs doesn’t reduce its effectiveness.
Post-Attachment Inhibitors
Ibalizumab (brand name Trogarzo) takes a different approach entirely. Rather than targeting the virus, it’s a monoclonal antibody that binds to a receptor on your own CD4 immune cells, changing the receptor’s shape so the virus can’t complete its entry into the cell. It’s given as an intravenous infusion: a one-time loading dose followed by maintenance infusions every two weeks. This is specifically approved for adults with multidrug-resistant HIV whose current regimen is failing.
The Role of Boosting Agents
Some HIV drugs are broken down by the body too quickly to maintain effective levels on their own. Pharmacokinetic enhancers, commonly called boosters, solve this problem by blocking the liver enzyme responsible for metabolizing the drug. This raises and sustains the active drug’s concentration in the blood.
Two boosters are currently used. Both powerfully inhibit the same liver enzyme, resulting in higher peak levels, greater overall drug exposure, and a longer effective duration for the partner medication they’re paired with. In the context of resistance, this matters because higher drug concentrations can sometimes overcome low-level resistance that would cause treatment failure at standard levels. For salvage regimens, boosted protease inhibitors remain one of the high-barrier drug options that guidelines rely on as anchors for new combinations.
Why Adherence Matters Even More
If inconsistent medication use contributed to resistance developing in the first place, the new regimen only works if that problem is addressed. This isn’t about willpower. Common barriers include side effects that make pills hard to tolerate, complicated dosing schedules, food requirements that are easy to forget, interactions with other medications, and life circumstances like housing instability or mental health challenges that make daily routines difficult.
For people with adherence challenges, the long-acting options described above can be especially valuable. A shot every six months or an infusion every two weeks removes the daily decision point entirely. When oral medications are part of the regimen, simplifying the pill burden, using single-tablet combinations where possible, and addressing side effects proactively all improve the odds of sustained suppression.
Drug level monitoring can also play a role in salvage therapy. By measuring the actual concentration of medication in the blood, clinicians can identify whether drug interactions, absorption problems, or other pharmacological factors are undermining the regimen, even when adherence is consistent. This is particularly useful when resistance test results and drug levels are interpreted together to fine-tune the combination.
What Successful Treatment Looks Like
The benchmark for any HIV regimen, including salvage therapy, is an undetectable viral load. For most people, this means fewer than 50 copies of the virus per milliliter of blood. Reaching this goal with a resistant virus may take longer than it did with initial treatment, and viral load is typically monitored more frequently during the transition to a new regimen.
Even when full suppression isn’t immediately achievable, keeping the viral load as low as possible protects the immune system and slows further resistance from developing. Partial suppression with the best available combination is still far better than no treatment, and as new drugs continue to reach approval, options that weren’t available during one regimen change may become available for the next.