Do some drugs develop resistance more easily?

Some drugs only need one mutation for the virus to be complete resistance. This is the case with NNRTIs and some nukes like 3TC and FTC. See Figure 4.

These drugs are more vulnerable if used in a combination that doesn’t keep your viral load below 50 c/mL.

They are also often cross-resistant to similar drugs in the same class.

Other drugs, including protease inhibitors, develop resistance more gradually.

The first mutations do not make much impact but as more complex resistance develops the drugs eventually stop working. See Figure 5. These drugs take longer to develop cross-resistance to other drugs in the same family.

Some nukes need only one mutation and some develop more complicated patterns of resistance.

Integrase inhibitors are more similar to NNRTIs in being vulnerable to resistance.

Figure 4: How one mutation can stop some drugs working

Graph showing 1 mutation causing extensive resistance.

Some drugs stop working after only one mutation.

These include NNRTIs (nevirapine, efavirenz, rilpivirine and etravirine), integrase inhibitors (raltegravir) and some nukes (3TC  and FTC).

Figure 5: Resistance increases slowly with some drugs

Graph showing number of mutations increasing from 1 (low resistance) to 6(extensive resistance)

With some other drugs, the first one or two mutations make little difference.

If you continue taking the same drug, more mutations will develop that eventually stop the drugs working. These include most PIs and some nukes.

Last updated: 1 January 2018.