HIV mainly infects cells in the immune system called CD4 cells. When someone has HIV, the number of CD4 cells drops gradually but continually and the immune system is weakened. HIV treatment works by interrupting this process. To stop HIV reproducing, it is necessary to take a combination of anti-HIV drugs that each target the virus in a slightly different way. These drugs are very effective in working against HIV, but they can also have unwanted side-effects.

It isn’t just anti-HIV drugs that cause side-effects. All medicines (including drugs bought without a prescription ‘over the counter’ and complementary and alternative medicines, which include so-called ‘natural’ and ‘herbal’ medicines) can have unwanted side-effects.

Side-effects can also happen because someone is taking several different drugs which interact with each other. It is important that your doctor and pharmacist know about all other medicines and drugs that you are taking – this includes those prescribed by another doctor, over-the-counter remedies, herbal and alternative treatments, and recreational drugs.

Every anti-HIV drug now in use has been through years of research. This includes clinical trials with hundreds, if not thousands, of people with HIV. These trials are run to make sure that the drugs work against HIV and are safe to use. For a drug to be approved for use it has to be shown that the benefits of using it outweigh the side-effects that it causes.

Most of the side-effects identified in clinical trials tend to be quite mild – for example, diarrhoea, feeling sick, or headache – and they are often temporary. But even mild side-effects can affect your quality of life, so it is important to know about the risk of them.

Sometimes more serious side-effects are identified in clinical trials and research. A particularly serious, even dangerous, side-effect can be an allergic reaction to a drug. This is rare, but it is important to know the signs of a reaction. You can find more detailed information on this in the section Hypersensitivity (allergic) reactions.