Side effects and drug levels

The occurrence of side effects is sometimes related to the levels of drugs in your blood.

Side effects and drug levels
When is therapeutic drug monitoring appropriate?

Side effects and drug levels

There can be large differences in levels of drugs absorbed between different people and by the same people at different times. Many different drug and food interactions can affect drug levels.

Side effects are often related to the level of a drug in your blood.

For some HIV drugs, these levels can be tested, and the dose can be changed if this is appropriate.

Protease inhibitors and NNRTIs are suitable drugs to measure
Nukes (d4T, AZT, 3TC, FTC, ddI, abacavir and tenofovir) are not suitable for drug level monitoring because the important levels of these drugs are inside cells and tests to measure this are not currently available

Some clinics do this test routinely but in others you may need to ask for it.

TDM is important for children and people with pre-existing liver or kidney damage ... and... whenever drug levels or drug interactions may be linked to side effects.

Drug interaction websites:

HIV Drug Interactions - University of Liverpool
www.HIV-druginteractions.org

HIV Pharmacology
www.HIVpharmacology.com

When is therapeutic drug monitoring (TDM) appropriate?

TDM usually involves taking a blood sample after you have been on a treatment for at least two weeks. The hospital will need to know the exact time that you took your previous dose in order for the test to be effective.

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 as well.

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

It is also appropriate whenever drug levels or drug interactions may be linked to side effects.

For example, the adult dose of atazanavir is 300mg plus 100mg ritonavir, both once daily. The ritonanvir boosts the levels of atazanavir and leaves a safer higher dose at the end of each 24-hour period.

If you get side effects with ritonavir boosting though, it may be possible to use a higher dose of atazanavir (400mg daily) and not use the ritonavir at all. This should always be checked with TDM.

TDM is available in the UK through programmes subsidised by the manufacturers of most PIs and NNRTIs.

For more information see:

Delphic Diagnostics
http://www.delphicdiagnostics.com