Fuzeon (generic name enfuvirtide) was the first of a new class of antiretroviral drugs, approved by the United States FDA on March 13, 2003, on the basis of two studies (TORO 1 and TORO 2) which compared the effect of optimized regimens of antiretroviral medication with and without the addition of Fuzeon on serum viral load.

The drug, in development since 1996, works by inhibiting the binding of HIV to the CD4 receptor on human cells, preventing uninfected cells from becoming infected. Drugs that work by this mechanism are termed entry inhibitors.

Initially designated T-20, (it was developed by Trimeris Inc.) the medication has several disadvantages that resulted in its slow progress. It is difficult to manufacture, expensive, and is only available in an injectable form. The injections must be taken twice a day.

Because of this, it is thought likely that the medication will be used as "salvage therapy": that is, as a drug to be used after other combinations of antiretroviral medications have failed.

The most common side effect is injection site reaction.

Trimeris partnered with Roche phamaceuticals to develop the drug in 1999. It is estimated that the U.S. price for a year of Fuzeon will be more than $25,000.