The anti-HIV drugs with the greatest risk of hypersensitivity, or allergy, are abacavir (Ziagen, also in the combination pills Kivexa and Triumeq) and nevirapine (Viramune). However, others may also cause an allergic reaction.

Abacavir can cause a serious hypersensitivity (allergic) reaction. This is associated with the presence of a particular gene called HLA-B*5701.

Before starting treatment with abacavir (or any treatment that contains abacavir) you should first have a blood test to see if you have this gene. If the test is positive you must not take abacavir. If the test is negative, it is highly unlikely that an allergic reaction will occur, but contact your HIV clinic immediately (or A&E if out of hours) if you begin to feel unwell after starting the drug.

The reaction can happen at any time while you are taking abacavir, but it is most likely to happen in the first six weeks after you start treatment with it.

In the box with the drug there is an ‘alert card’, which you should carry with you for the first six weeks of taking abacavir.

The most common symptoms are fever (high temperature) and skin rash.

Other common symptoms are:

• nausea (feeling sick)
• vomiting (being sick)
• diarrhoea
• stomach pain
• severe tiredness.

Other symptoms include pains in the joints or muscles, swelling of the neck, shortness of breath, sore throat, cough, occasional headaches, inflammation of the eye (conjunctivitis), mouth ulcers, low blood pressure, and tingling or numbness of the hands or feet.

A very rare hypersensitivity (allergic) reaction has been reported in some people using other anti-HIV drugs, including atazanavir, etravirine, efavirenz, darunavir, maraviroc and raltegravir. This can cause a severe rash, called Stevens Johnson syndrome. See your HIV clinic immediately (or A&E if out of hours) if you develop a rash together with any of these symptoms:

• fever
• feeling generally unwell or extremely tired
• muscle or joint ache
• blistering of the skin
• mouth ulcers
• swelling of the eye, lips, mouth or face
• breathing difficulties
• yellowing of the skin or eyes
• dark urine
• pale stools
• pain, aching or sensitivity on the right-hand side of the body, below the ribs.

If you have this sort of reaction, you should not take the drug which caused it again in the future.

It’s possible that other drugs, including other anti-HIV drugs, might rarely cause allergic reactions too. It is therefore important that you tell your doctor as soon as possible if you develop a rash, fever, or feel generally unwell soon after starting treatment with a new drug.