Tecovirimat, sold under the brand name TPOXX, is the first antiviral drug approved specifically for treating smallpox and the primary antiviral used during recent mpox outbreaks worldwide. It works by targeting a protein that poxviruses rely on to move from cell to cell, effectively crippling the virus’s ability to spread through the body. The drug is generally well tolerated, but recent large clinical trials have raised hard questions about how well it actually works against mpox, making the story of TPOXX more nuanced than its early promise suggested.
How Tecovirimat Works
Poxviruses, including the viruses that cause smallpox and mpox, produce a protein called VP37 (encoded by a gene known as F13L in mpox virus). VP37 sits on the surface of newly formed viral particles and helps them wrap themselves in a membrane, which the virus then uses to exit the cell and infect neighboring cells. Without that wrapping step, viral particles get stuck inside the host cell and can’t spread efficiently.
Tecovirimat binds directly to VP37 and blocks its function. The virus can still replicate inside an infected cell, but the new copies largely stay trapped there. This gives the immune system time to catch up, clearing the infection before it can cause widespread tissue damage. Because VP37 is shared across orthopoxviruses, tecovirimat has activity against not just variola (the smallpox virus) but also monkeypox virus, cowpox, and vaccinia.
An Unusual Approval Path
Smallpox was eradicated in 1980, which creates an obvious problem for drug development: you cannot run a clinical trial in humans for a disease that no longer exists naturally. Tecovirimat was approved in 2018 under the FDA’s Animal Rule, a regulatory pathway that allows marketing approval based on efficacy demonstrated in relevant animal models when human trials are ethically or practically impossible.1PubMed Central. The development and approval of tecoviromat (TPOXX®), the first antiviral against smallpox In studies using non-human primates and rabbits infected with lethal orthopoxviruses, tecovirimat dramatically improved survival, even when treatment was started several days after infection.2PubMed. Tecovirimat: First Global Approval
Human safety data, however, did come from healthy volunteers. Those early studies established that the drug was safe and well absorbed, providing the safety half of the regulatory package while animal data supplied the efficacy half. This approval was always understood as a biodefense measure: TPOXX was stockpiled by the U.S. Strategic National Stockpile as a countermeasure against a potential smallpox bioterrorism event, not something anyone expected to prescribe routinely.
When Tecovirimat Is Used
The emergence of mpox as a global concern changed the drug’s profile overnight. When the multi-country clade II mpox outbreak began in 2022, the existing expanded access investigational new drug (EA-IND) protocol, which already permitted tecovirimat use for non-variola orthopoxvirus infections, allowed clinicians to start prescribing it almost immediately.3PubMed Central. Tecovirimat Use under Expanded Access to Treat Mpox in the United States, 2022–2023 Tecovirimat became the first-line antiviral treatment recommended for severe mpox and for patients at higher risk of severe disease.4MMWR Morbidity and Mortality Weekly Report. Successful Distribution of Tecovirimat During the Peak of the Mpox Outbreak — Los Angeles County, June 2022–January 2023
In the United States, tecovirimat is still available under expanded access IND protocols rather than a full FDA approval for mpox. This means clinicians prescribing it for mpox must follow specific reporting requirements, including documenting adverse events. In practice, it has been widely dispensed during outbreaks, with tens of thousands of treatment courses distributed during the 2022–2023 surge alone. For smallpox, the drug carries full FDA approval and sits in the Strategic National Stockpile, ready for deployment if the variola virus were ever released deliberately.
Dosage and How It Is Given
Tecovirimat comes in two formulations: oral capsules and an intravenous (IV) solution. The standard adult oral dose for patients weighing 40 kilograms or more is 600 mg taken twice daily with food, continued for 14 days.5PubMed Central. Tecovirimat for the Treatment of Mpox Taking the capsules with a full meal substantially improves absorption; without food, blood levels of the drug drop significantly, which can undermine its effectiveness.
The IV formulation exists for patients who cannot swallow or absorb oral medication, such as those with severe mucosal lesions in the mouth or throat, or critically ill patients in intensive care. Because tecovirimat does not dissolve easily in water, the IV version uses a solubilizing agent called hydroxypropyl-beta-cyclodextrin. Drug labeling recommends caution in patients with kidney impairment due to that solubilizer, though studies of other IV drugs using the same agent have not shown significant kidney toxicity.6PubMed Central. Antivirals With Activity Against Mpox: A Clinically Oriented Review In practice, clinicians often monitor kidney function during IV courses and adjust as needed.
For younger children who cannot swallow capsules, researchers have shown that the 200 mg capsule can be opened and dispersed in 20 milliliters of water, swirled for 30 to 60 seconds, to create a consistent 10 mg/mL suspension suitable for oral dosing.7PubMed Central. Mixing of Tecovirimat in Water to Support Oral Dosing of Infants and Children With Mpox Pediatric dosing is weight-based, with lower total daily amounts for smaller children, scaled to match the drug exposure seen in adults.
Side Effects
Tecovirimat has been consistently described as well tolerated across studies and compassionate-use reports. In one early compassionate-use series of 25 patients treated for mpox, no one discontinued therapy due to side effects. The most commonly reported adverse events during treatment were fatigue (about 28% of patients), headache (20%), nausea (16%), itching (8%), and diarrhea (8%).8JAMA. Compassionate Use of Tecovirimat for the Treatment of Monkeypox Infection These side effects were generally mild and self-limiting.
Liver enzyme elevations (ALT and AST) have occasionally been noted during tecovirimat courses, though in at least some cases the rises were more likely related to concurrent antibacterial treatment or the infection itself rather than tecovirimat.9PubMed Central. A severe monkeypox infection in a patient with an advanced HIV infection treated with tecovirimat: clinical and virological outcome Serious adverse events attributable to the drug have been rare in published case series and trial data. For most patients, the side-effect profile compares favorably with many common antivirals.
What Clinical Trials Found About Efficacy
Here is where the TPOXX story gets complicated, and where the gap between lab promise and clinical reality has proven wider than many hoped.
The largest trial for clade II mpox, the international STOMP trial, was a phase 3, double-blind, randomized, placebo-controlled study in adults with confirmed mpox. Participants received either tecovirimat or placebo for 14 days, and the primary outcome was time to clinical resolution of skin and mucosal lesions.5PubMed Central. Tecovirimat for the Treatment of Mpox The results were sobering: tecovirimat did not significantly speed up lesion resolution compared to placebo. Secondary endpoints, including pain reduction, complete lesion healing, and viral DNA clearance, also failed to show a clear benefit. In short, for the relatively mild clade II mpox that drove the 2022 global outbreak, the drug did not perform meaningfully better than supportive care alone.
A separate randomized trial evaluated tecovirimat against clade I mpox in the Democratic Republic of Congo, where clade I causes more severe illness and higher mortality. This trial, known as PALM007, also used a double-blind, placebo-controlled design in patients with at least one confirmed mpox lesion.10PubMed. Tecovirimat for Clade I MPXV Infection in the Democratic Republic of Congo Its primary endpoint was resolution of mpox lesions measured in days after randomization. Disappointingly, this trial also did not demonstrate a significant benefit for tecovirimat over placebo in lesion resolution time.
These results do not necessarily mean the drug is useless. Some researchers point out that the trials enrolled patients who were, on average, presenting relatively early in their illness and were otherwise healthy enough to participate. Whether tecovirimat might benefit the sickest patients, particularly those with advanced immunosuppression or complications like encephalitis, remains an open question that randomized trials have not yet answered. Individual compassionate-use reports have described favorable outcomes in severely ill patients, but those observations carry much less weight than controlled trial data.
Who Is Considered High Risk
Even with the uncertain efficacy picture, clinical guidelines still recommend tecovirimat for people at elevated risk of severe mpox. These groups include people with advanced HIV infection or other forms of immunosuppression, children (especially young children), and pregnant individuals.9PubMed Central. A severe monkeypox infection in a patient with an advanced HIV infection treated with tecovirimat: clinical and virological outcome The reasoning is straightforward: in patients whose immune systems cannot reliably clear the virus on their own, blocking viral spread with an antiviral may provide a benefit that the trials, which enrolled mostly immunocompetent adults, could not detect.
Case reports of tecovirimat use in severely immunocompromised patients, including those with advanced HIV, have generally shown good tolerability and, in some cases, viral clearance that coincided with treatment. But the evidence base for these populations remains anecdotal rather than trial-driven. Clinicians typically weigh the low risk of side effects against the potentially serious consequences of untreated severe mpox and decide to treat.
Resistance Is a Real and Growing Concern
Like all antivirals, tecovirimat faces the problem of resistance. The drug’s target, the F13 protein, is encoded by the F13L gene in mpox virus. Single amino acid changes in that protein can reduce or eliminate the drug’s ability to bind, rendering it ineffective.11PubMed Central. Tecovirimat Resistance in Mpox Patients, United States, 2022-2023 Cases of resistance emerged during the 2022–2023 mpox outbreak in the United States, particularly in patients who received prolonged courses or had ongoing viral replication due to immunosuppression.
One mutation that has attracted particular attention is G277C in the F13L gene, which has been shown to significantly increase resistance to tecovirimat.12PubMed Central. Structural and dynamic insights into F13L variations in monkeypox virus and their impact on tecovirimat resistance This is especially worrying because tecovirimat is currently the only antiviral with a specific FDA approval pathway for orthopoxvirus infections. If widespread resistance developed, the options would narrow considerably.
The risk of resistance underscores the importance of not treating tecovirimat as a universal solution. In immunocompromised patients who may harbor replicating virus for weeks or months, prolonged antiviral courses create exactly the selective pressure that favors resistant mutants. Genomic surveillance of mpox isolates from treated patients is becoming a standard part of outbreak response, but capacity for that surveillance varies widely between countries.
Other Antivirals in the Picture
Tecovirimat is not the only drug with activity against orthopoxviruses. Two other antivirals, cidofovir and brincidofovir, also inhibit poxvirus replication, though through a different mechanism: they interfere with the virus’s DNA replication rather than its cell-to-cell spread.6PubMed Central. Antivirals With Activity Against Mpox: A Clinically Oriented Review Cidofovir is given intravenously and carries a significant risk of kidney toxicity, which limits its use. Brincidofovir, an oral prodrug of cidofovir designed to reduce kidney problems, received FDA approval for smallpox in 2021, also under the Animal Rule, but has its own tolerability issues including liver enzyme elevations and gastrointestinal side effects.
In practice, these alternatives are generally reserved for situations where tecovirimat cannot be used or where resistance has been documented. Some clinicians have considered combination approaches, pairing tecovirimat with cidofovir or brincidofovir to attack the virus at two different points in its life cycle and potentially reduce the chance of resistance. Formal clinical trial data supporting combination therapy in humans remains sparse, but the concept follows established principles from treating other viral infections like HIV and hepatitis C, where multidrug regimens are standard specifically to prevent resistance.
Post-Exposure Use
Beyond treating active infections, tecovirimat has been explored as a post-exposure prophylactic, meaning a drug you take after a known exposure to prevent disease from developing. In animal models, administering tecovirimat within 4 to 72 hours after poxvirus exposure prevented death and reduced the severity of skin lesions.6PubMed Central. Antivirals With Activity Against Mpox: A Clinically Oriented Review
Human data on post-exposure prophylaxis with tecovirimat is limited. During the 2022 mpox outbreak, some clinicians offered it to close contacts of confirmed cases, particularly those at high risk for severe disease. Whether this meaningfully reduced transmission or prevented illness in those individuals is difficult to assess outside of a controlled trial. The animal data is encouraging, but the window of effectiveness and the optimal dosing regimen for human post-exposure use have not been formally established. For the moment, vaccination with MVA-BN (marketed as Jynneos) remains the primary post-exposure measure recommended by public health agencies, with tecovirimat considered supplementary in certain high-risk scenarios.
Global Access Remains Deeply Unequal
One of the sharpest criticisms surrounding tecovirimat has nothing to do with its pharmacology and everything to do with who can get it. The drug was developed as a U.S. biodefense product, manufactured by SIGA Technologies under government contract, and primarily stockpiled in wealthy nations. During the 2022 outbreak, cities in the United States and Europe were distributing courses of TPOXX within days of the outbreak’s acceleration, while countries in sub-Saharan Africa, where clade I mpox has been endemic for decades and causes higher mortality, had little to no access.13Current Topics in Chemistry. Advancing Antiviral Therapeutic Strategies: Addressing the Global Health Challenge of the Monkeypox Outbreak
The disparity is not just a supply-chain problem. The expanded access IND protocol that enabled rapid prescribing in the United States requires data collection infrastructure, regulatory engagement with the FDA, and clinical capacity that many endemic-region health systems do not have. Even when donated supplies reach affected countries, the logistics of cold-chain storage, clinical training, and follow-up reporting can delay or limit distribution. Advocacy groups have called for generic manufacturing agreements and technology transfer to allow lower-cost production closer to where the burden of disease is highest, but as of mid-2025 those efforts are still in early stages.
Drug Interactions Worth Knowing About
Tecovirimat is metabolized primarily through the liver, and it can interact with other drugs that are processed by the same enzyme pathways. One interaction that gets clinical attention involves repaglinide, a diabetes medication: tecovirimat can increase repaglinide blood levels, potentially causing dangerously low blood sugar. The FDA labeling warns against co-administration. Other potential interactions involve drugs metabolized by certain liver enzymes, and clinicians are advised to review a patient’s full medication list before prescribing.
For people living with HIV, the interaction profile is particularly relevant since many antiretroviral drugs are also metabolized by liver enzymes. Published case reports of tecovirimat use in patients on antiretroviral therapy have not flagged major interaction-driven adverse events, but pharmacokinetic studies specifically examining the combinations are limited. In practice, clinicians managing mpox in people with HIV tend to monitor closely rather than withhold tecovirimat, given the seriousness of the underlying infection.
The Gap Between Bench and Bedside
Tecovirimat presents an unusual case study in antiviral development. In laboratory assays, the drug is potent against orthopoxviruses, with very low concentrations needed to block viral spread in cell culture. In animal models of lethal poxvirus infection, it is unambiguously life-saving. Yet in the two largest human randomized trials for mpox, it failed to outperform placebo on the primary endpoint of lesion resolution. Understanding why requires some nuance about the disease itself.
Clade II mpox in immunocompetent adults is, for most people, a self-limiting illness. Lesions progress through defined stages and resolve on their own over two to four weeks. A drug that blocks late-stage viral spread may simply not have much room to improve outcomes in a disease that the immune system is already handling. The patients who might benefit most, those with profoundly weakened immune systems or with complications affecting the eyes, brain, or lungs, are exactly the patients most difficult to enroll in large numbers in a randomized trial. This mismatch between the population studied and the population most likely to benefit has left clinicians in a difficult position: the drug’s safety profile is reassuring, the animal data is strong, but the human evidence says it does not clearly help the average mpox patient.
Whether future trials targeting severely immunocompromised patients or testing earlier intervention will change the picture remains to be seen. In the meantime, TPOXX continues to be used under expanded access for high-risk patients, reflecting a pragmatic calculation that in the absence of better options, a safe drug with strong preclinical data is preferable to no antiviral at all.