How do missed doses lead to resistance?

Resistance and adherence are closely related. If you miss doses or are late taking your meds, this increases the chance of resistance. See Figures 2 and 3 below.

This is because drug levels fall below the minimum needed to control the virus.

The mutations that occur when you have only low concentrations of your drugs can stop the drugs working. Then, when you restart or continue treatment, they may not work at all.

Adherence is just as critical when you are on your second, third or later combination.

Figure 2: Drug levels with good adherence

Drug doses are calculated on average levels over the whole dose period.

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

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

Figure 3: A missed or late dose increases the risk of resistance

Missing or being late with a drug lets the drug levels fall below a minimum safe level.

Drug resistance can then develop.

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