Ziagen (abacavir) is an antiretroviral medicine used as part of combination treatment for HIV-1 infection in adults and children. It belongs to the nucleoside reverse transcriptase inhibitor (NRTI) class and works by blocking the reverse transcriptase enzyme that HIV needs to copy its genetic material into human cells.

Abacavir is also a component of several combination tablets: Triumeq (abacavir/dolutegravir/lamivudine) and Kivexa (abacavir/lamivudine). These fixed-dose combinations reduce pill burden.

Ziagen can be taken with or without food. The standard adult dose is 600 mg once daily (as two 300 mg tablets) or 300 mg twice daily.

Take at the same time(s) each day. Ziagen must always be used in combination with other antiretroviral medicines.

Before starting Ziagen, you must be tested for the HLA-B*5701 gene variant. This is essential: see below.

Take the missed dose as soon as you remember. If it is nearly time for the next dose, skip and continue your schedule. Never take a double dose.

Contact your HIV specialist or a poison control centre.

• Tablets: store below 30°C
• Oral solution: store below 25°C. Can be refrigerated but do not freeze
• Keep out of reach of children

Most people tolerate Ziagen well once the HLA-B*5701 test is confirmed negative. However, one serious side effect requires vigilance in all patients:

Abacavir hypersensitivity reaction (HSR):

This is the most important side effect of abacavir. Even though HLA-B*5701 testing identifies most people at risk, a very small number of HLA-B*5701-negative people may still develop HSR.

Symptoms typically develop within the first six weeks of treatment and involve multiple organ systems:

• Fever
• Rash (may or may not be present)
• Gastrointestinal symptoms: nausea, vomiting, diarrhoea, or abdominal pain
• Fatigue and malaise
• Respiratory symptoms in some patients

If you develop two or more of these symptoms together within the first six weeks, stop Ziagen immediately and contact your HIV doctor. Never restart abacavir after a suspected hypersensitivity reaction, as re-challenge can cause a rapid, life-threatening reaction.

Other common side effects:

• Nausea
• Headache
• Fatigue
  

Long-term effects:

• Lactic acidosis: rare but serious. Report unusual fatigue, breathing difficulties, or stomach pain
• Cardiovascular risk: some studies suggest a possible association between abacavir use and cardiovascular events in certain high-risk patients. Discuss with your HIV doctor

• Methadone: abacavir slightly reduces methadone levels. Dosage may need adjustment
• Alcohol: increases abacavir exposure slightly. Not a strict contraindication, but worth noting
  

Tell your HIV doctor and pharmacist about all medicines you take.

What is the HLA-B*5701 test?

HLA-B*5701 is a genetic variant (allele) found in a small proportion of the population. People who carry this variant are at very high risk of a severe hypersensitivity reaction to abacavir. The test is a simple blood test done before prescribing any abacavir-containing medicine. If you test positive for HLA-B*5701, you cannot take Ziagen or any abacavir-containing combination. If negative, abacavir is generally safe, but you must still watch for hypersensitivity symptoms in the first six weeks.

What should I do if I think I'm having a hypersensitivity reaction?

Stop Ziagen immediately and contact your HIV doctor or emergency department. Do not wait to see if it improves. Describe all your symptoms. If the reaction is confirmed, never take abacavir again. Even a brief re-exposure after a genuine HSR can trigger an immediate, severe, and potentially life-threatening reaction.

Ace provides accurate and independent information medically reviewed on prescription medications. This material is provided for educational purposes only and is not intended for medical advice, diagnosis or treatment.