The two anti-HIV drugs with the biggest risk of allergy are abacavir (Ziagen, also in the combination pills Kivexa and Trizivir) and nevirapine (Viramune).

It is thought that about 8% of people who start treatment with abacavir will develop an allergic reaction to the drug (this is often called an abacavir hypersensitivity reaction). It is linked to a gene called HLA-B*5701 and you should have a blood test to see if you have this gene before you start treatment with abacavir.

If your test for the gene is positive you should not take abacavir or any of the combination pills that include abacavir. If your test result is negative, you can start treatment with abacavir, but should tell your doctor immediately if you develop symptoms such as a fever, rash, nausea, vomiting or abdominal pain as you may still develop an allergic reaction, although the risk of this is small. If you stop taking abacavir because you have had an allergic reaction to the drug you must never take abacavir again as this can be very dangerous, even fatal.

A very small number of people who start treatment with nevirapine develop serious liver problems. This reaction also appears to be related to a particular gene. To reduce the risk of an allergic reaction to nevirapine, men with a CD4 cell count above 400 should not start treatment with nevirapine, nor should women with a CD4 cell count of more than 250.

It’s possible that other drugs, including 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 or fever, or feel generally unwell soon after starting treatment with a new drug.