Do some drugs develop resistance more easily?

Some drugs only need one mutation for the virus to become completely resistant to them. This is the case with NNRTIs (nevirapine and efavirenz) and some nukes (3TC and FTC) – see Figure 4.

Figure 4: How one mutation can stop some drugs working

Some drugs stop working after only one mutation. These include NNRTIs (nevirapine and efavirenz) (NNRTIs), some nukes (3TC and FTC).

These are potent drugs but they are more vulnerable to early failure if used in a combination that does not reduce your viral load to below 50 copies/mL.

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

Other drugs, including protease inhibitors, develop resistance more gradually – see Figure 5 .

The first few mutations do not have much impact but they are the pathway to more complicated resistance. It then usually takes several mutations to stop the drug from working below.

Figure 5: Resistance increases slowly with some drugs

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.

Drugs that require accumulated resistance take longer to develop cross-resistance to other drugs in the same family.

Nukes can vary – some need only one mutation and some develop more complicated patterns of resistance.

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

• The i-Base Guide to HIV Drug Resistance explains drug resistance, resistance tests and understanding the results in detail.
• Resistance and adherence are discussed in detail in our Introduction to Combination Therapy.