The occurrence of side effects is sometimes related to the levels of drugs in your blood. These levels can be tested, for some HIV drugs.

There can be large differences in levels of drugs absorbed between different people. There are many different interactions that can also affect drug levels.

Although you need to reach a certain minimum level for the drugs to work properly, some people have much higher levels.

Higher concentrations of drugs often provide a stronger anti-HIV effect, so it is important to get the balance right.

Protease inhibitors and NNRTIs are suitable drugs to measure.

The nucleoside analogues (d4T, AZT, 3TC, ddI and abacavir) are not appropriate for drug level monitoring.

Therapeutic Drug Monitoring (TDM) usually involves taking a series of blood samples (after you have been on a treatment for at least two weeks).

TDM is most likely to be helpful –

TDM is important for children and people with pre-existing liver damage for whom routine recommended dosing is not always appropriate.

Checking your own combination makes a lot of sense – and TDM is available in the UK, through a subsidised programme by Roche (for nelfinavir and saquinavir), Merck (for indinavir) and GSK (for amprenavir).