The commonest side-effect of d4T (stavudine, Zerit) is peripheral neuropathy. This is nerve damage in the feet, legs and hands, which can cause numbness, tingling or pain in the extremities. The pain associated with neuropathy can be severe.

Peripheral neuropathy occurs in 15 to 21% of people on d4T, particularly in those on higher doses, with more advanced HIV disease, or who are also taking ddI (didanosine, Videx / VidexEC). If d4T is stopped, neuropathy usually gets better after about two weeks. If symptoms disappear, d4T can be restarted at half the recommended dose. Recent studies have also shown that halving or reducing the dose of d4T by 10mg has no effect on its efficacy but can reduce the incidence of peripheral neuropathy^[1][2][3].

Due to its tendency to make peripheral neuropathy worse, d4T is usually not recommended to people with pre-existing neuropathy.

Body fat changes known as lipodystrophy are also a major concern with d4T. Although the syndrome was originally linked with protease inhibitors, a number of studies have found an association between lipodystrophy and the nucleoside reverse transcriptase inhibitors, with the strongest association observed with d4T^{[4][5][6][7][8]}. This association is also found in children and adolescents^[9]. Consequently, the British HIV Association recommends that d4T should not be used in first-line therapy.

There is evidence that fat loss stops after the dose of d4T is reduced by 10mg or it is stopped altogether^{[10][11][12][13]}. Switching from d4T to abacavir (Ziagen) can also help to reverse fat loss^[14][15].

Risk factors for d4T-related fat loss include:

• Being over 40 years old.
• Having high baseline triglyceride levels (over 200mg/dl).
• Using standard immediate-release d4T: although not yet routinely available, the new once daily extended-release (XR) version of the drug produces a slightly lower incidence of lipoatrophy over 48 weeks^[16].

Fat loss is thought to be due to damage to the mitochondria within fat cells, although d4T has been shown to reduce levels of mitochondrial DNA in fat cells long before fat loss is clinically apparent^[17][18]. For more information, see Possible causes of body fat and metabolic changes.

Elevated lactate and other metabolic abnormalities have also been linked to d4T treatment. Lactic acidosis is a rare but serious side-effect of d4T causing symptoms such as nausea, malaise and liver pain, which can lead to eventual death if nucleoside reverse transcriptase inhibitor (NRTI) treatment is not stopped. Of all the NRTIs, d4T has been particularly associated with elevated lactate levels. It usually develops within the first few months of d4T treatment. Pregnant women taking d4T and ddI (didanosine, Videx / VidexEC) may be at increased risk of lactic acidosis, as may patients with hepatitis C who are taking interferon alfa (IntronA / Roferon-A / Viraferon) and ribavirin (Copegus / Rebetol / Virazole). Some experts believe that this is caused by d4T damaging the mitochondria, the subcellular compartments that provide energy for the cell, causing a range of side effects including fat loss and peripheral neuropathy. See Mitochondrial toxicity for more details.

Around 2% of people taking d4T develop pancreatitis, although the risk of this occurring is greater in patients who have had pancreatitis in the past. Elevated amylase levels are common, and can be a warning sign for increased risk of pancreatitis.

Other side effects of d4T are most likely to occur during the early weeks of treatment. These include nausea, vomiting, listlessness, chills, fever, diarrhoea, constipation, headaches, abdominal pain and dehydration. Medicines to control nausea and diarrhoea can be prescribed before a patient starts d4T.

Other less common toxicities of d4T include elevated liver enzymes and fatty liver, particularly in people with hepatitis B or C co-infection^[19]. There have also been anecdotal reports of sleep disorders, mania and skin rashes, as well as elevations of urate levels in the blood, which can lead to gout and kidney problems^[20].