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Immune & defense

Enfuvirtide

Also called T-20

EvidenceFDA approved

ClassHIV fusion inhibitor peptide

Enfuvirtide (Fuzeon) blocks HIV from entering CD4 cells. See how this fusion inhibitor works, who used it, its 2003 FDA approval, and its 2025 discontinuation.

Reviewed by: William Maish, MD MBA MPH, Clinical Product Lead
Published: October 5, 2026
Last updated: October 5, 2026

Key takeaway

Enfuvirtide (formerly sold as Fuzeon) is a peptide HIV drug that blocks the virus from entering CD4 immune cells, the first of the so-called fusion inhibitors. It was used in treatment-experienced patients whose virus is no longer controlled by other antiretrovirals. It was FDA-approved in the U.S. in 2003, but Fuzeon was discontinued there in February 2025.

At a glance

What it is	An HIV-1 fusion (entry) inhibitor, a 36-amino-acid peptide
Approved for	HIV-1 infection in treatment-experienced patients, in combination with other antiretrovirals (approved in 2003; no longer sold in the U.S.)
Key fact	In the TORO trials, adding enfuvirtide to an optimized regimen significantly lowered viral load versus the regimen alone
Route	Subcutaneous injection, twice daily
Evidence strength	FDA approved

What is enfuvirtide?

Enfuvirtide, formerly sold under the brand Fuzeon, is a 36-amino-acid synthetic peptide that blocks HIV from entering host cells. It was the first in a class called fusion inhibitors (or entry inhibitors) and is used in people whose HIV has become resistant to multiple other drugs. It was developed under the code T-20 and approved in the United States in 2003.

How does enfuvirtide work?

Enfuvirtide blocks the step where HIV fuses its envelope with the membrane of a CD4 immune cell to get inside. The viral surface protein gp41 must fold on itself, forming a hairpin-like structure, to pull the two membranes together. Enfuvirtide mimics a segment of gp41 and binds the region the protein needs to grip, preventing that folding. Without it, the virus cannot complete fusion and cannot enter the cell.

Because it works outside the cell at the point of entry, its mechanism is entirely different from the drugs that act inside the cell on reverse transcriptase, protease, or integrase, which is why it retains activity against virus resistant to those classes. In the TORO trials, adding enfuvirtide to an optimized background regimen produced a significantly greater reduction in HIV-1 RNA¹ than the background regimen alone. It must be injected twice daily, which, with frequent injection-site reactions, limits its use to situations where other options are exhausted.

Is it FDA-approved?

Enfuvirtide was approved by the FDA in 2003 for HIV-1 infection in treatment-experienced patients with ongoing viral replication despite antiretroviral therapy. Its maker discontinued² Fuzeon in the U.S. on February 28, 2025, citing reduced need as newer treatments became available.

What is enfuvirtide prescribed for?

Enfuvirtide was prescribed, in combination with other antiretrovirals, for HIV-1 infection in treatment-experienced patients whose virus was replicating despite current therapy. It was generally reserved for people with multidrug-resistant HIV because of the twice-daily injections.

What are the side effects?

Common side effects include:

Injection-site reactions such as pain, hardened lumps, redness, and cysts, which occur in nearly all patients
Diarrhea and nausea
Fatigue

More serious considerations include:

An increased rate of bacterial pneumonia
Hypersensitivity reactions, which can recur if the drug is restarted

More for immune & defense

Sources: FDA prescribing information for Fuzeon; the TORO 1 trial (Lalezari et al., The New England Journal of Medicine, 2003, DOI: 10.1056/NEJMoa035026¹), indexed on PubMed. This document is educational and does not constitute medical advice.