• Poor treatment adherence is a major cause of poor drug levels.
  • Interactions between drugs can influence drug levels.
  • Some people's bodies get rid of drugs faster than others.
  • Low drug levels in the blood may cause treatment to fail.
  • A small number of treatment centres are testing drug levels in people taking protease inhibitors.
  • Higher drug levels may mean greater anti-HIV activity but more severe and more frequent side-effects.