Side effects, drug levels and genetics

Most drugs are approved at one standard dose even though different people absorb drugs differently. This can be related to differences in our genes and is a new area of research called pharmacogenetics.

For example, tiny differences in your DNA can explain the differences in levels of drugs including efavirenz, nevirapine and atazanavir.

Just as the blood levels of a drug affect how effective it is, they also affect the chance of side effects.

Drug levels for some HIV drugs can be checked using a test called therapeutic drug monitoring (TDM). The dose can then be changed if they are too high or too low.

  • Protease inhibitors, NNRTIs and integrase inhibitors can be measured.
  • Nukes (3TC, FTC, abacavir, tenofovir DF and TAF) can not be measured. This is because the important levels of these drugs are inside cells and the tests measure drug levels in blood.

TDM is not routinely used but it is important in some situations.

When TDM might be considered

TDM can be important when routine dosing is appropriate.

For example:

  • In children.
  • In people with pre-existing liver or kidney damage.
  • When drug levels might be linked to side effects. If you get yellow eyes with atazanavir TDM can help find an effective lower dose.
  • When drug interactions are a concern. For example, when antacid drugs like omeprazole reduce levels of atazanavir and cause treatment to fail.

TDM involves taking a blood sample, usually after you have been on a treatment for at least two weeks.

The hospital needs to know the exact time that you took your previous dose in order to interpret the results.

Sometimes a sample is taken just before you are due to take your next dose, and sometimes it is also taken 2–3 hours afterwards.

TDM is part of an individualised approach for specific groups of people on ART.

More information on TDM

  • Lab 21 Commercial lab in the UK running TDM)

Drug interaction websites

Last updated: 1 November 2021.