Guides

Why adherence is linked to drug resistance

The doses of HIV drugs are worked out to give you effective drug levels for 24 hours a day.

These levels should be high enough to be active and not get resistance. They should not be so high that they cause side effects. See Figure 6.

Figure 6: Drug levels with good adherence

Graph showing drug levels staying within a band where there is less risk of side effects (high levels) and resistance (low levels).

Drug doses are worked out to cover the whole dose period. 

Levels need to be high enough to be active against HIV without risking resistance.

Levels need to be low enough to minimise the risk of side effects.

But HIV drugs can only work if you take them.

Missing doses or being late taking your meds increases the chance of drug resistance. See Figure 7.

Figure 7: A missed or late dose increases the risk of resistance

Graph showing how a missed or late dose causes drug levels to drop to a level with risk of resistance

Missing doses or being late taking your meds lets the drug levels fall below the minimum level needed. Drug resistance can then develop.

The more often you are late or miss a dose, the greater the chance that resistance will occur.

Drug resistance will stop treatment working. Resistance changes your virus forever. It reduces your choice of drugs.

This can affect current meds and new drugs in the future.

What is HIV drug resistance?

Drug resistance can explain why a drug no longer works.

Resistance can develop to drugs used to treat viral, bacterial and fungal infections.

The drugs stop working because the virus (or other organism) has evolved or changed while on treatment.

The risk of drug resistance increases when drug levels are low.

Resistance and adherence are closely related. If you miss doses or are late taking your meds, this increases the chance of resistance. 

This is because when you are late, drug levels can fall below the minimum level needed to control the virus.

The mutations that develop when you have only low levels of meds can stop the drugs working. Then, when you restart treatment, they may not work as well.

Adherence is even more critical when you are on your second, third or later combination as you have fewer drugs left to use.

Last updated: 1 January 2018.