Once swallowed, anti-HIV drugs pass through the digestive system where they are absorbed into the blood stream and distributed throughout the body. The rate at which they are absorbed varies between individuals. This means that if two people take identical treatment at the same doses and with the same foods, the amount of drug which will reach their blood streams can be very different.

To a certain degree, this variability is unimportant. In order to be effective against HIV, antiretrovirals must reach a level in the blood which falls within a range that is established when new drugs are first developed. A blood level which is higher than this 'therapeutic range' can lead to more side-effects. A lower level will allow ongoing HIV replication, which provides the circumstances for drug resistance to develop, causing the treatment to fail.

Drug levels reach their peak soon after they are taken, and then taper off over the subsequent hours to a lower 'trough level' before the next dose. It is this trough level which is likely to be pivotal in determining a drug's efficacy.