The most common side-effect associated with T-20 (enfuvirtide, Fuzeon) is the development of reactions around the injection site. In the TORO studies, 98% of patients who took T-20 got a site reaction. In the two studies, 46 to 53% of T-20 users reported mild tenderness at the injection site at any clinic visit, 15 to 19% reported moderate pain, and 1 to 3% reported severe pain, but only 4% stopped treatment because of the reaction^[1].

There seems to be less risk of injection site reaction when administration is via injection into the arm^[2]. However, the reactions are more severe in patients with less fat under the skin: this may be problematic for people with lipoatrophy following extensive antiretroviral therapy^[3].

Site reactions follow a number of patterns:

• Itchy rash.
• Red swollen or puffy skin.
• Hardened skin.
• Cysts or nodules forming at the injection site. These may be more common if injection occurs in the legs.

More rarely, several individuals have developed abscesses at the injection site. Patients usually experience only one type of site reaction over time.

The rashes and reactions may occasionally require treatment with anti-histamines or painkillers.

Other side-effects which may be due to T-20 include headache, insomnia, peripheral neuropathy, and eosinophilia (an increase in the number of immune cells known as eosinophils).

At the time of approval, licensing authorities in the United States and Europe drew attention to a small number of cases of hypersensitivity, characterised by rash, fever, nausea and vomiting. Two reactions occurred in the TORO 1 study. In both cases, the individuals affected were also taking amprenavir (Agenerase), a drug known to cause hypersensitivity reactions, although hypersensitivity recurred upon re-challenge with T-20.

Bacterial pneumonia and lymphadenopathy (swollen glands) also occur more frequently in T-20 users^[4]. T-20’s manufacturer, Roche has promised to explore these effects further in post-marketing studies. Bacterial pneumonia occurred in 7% of T-20 recipients in the TORO studies, compared to 1% of the control arm. Lymphadenopathy may be a result of the large amount of foreign protein injected by T-20 users over time, since T-20 is a peptide.