As repeated doses of a drug are administered its plasma concentration builds up and reaches what is known as a steady state. This is when the amount of drug in the plasma has built up to a concentration level that is therapeutically effective and as long as regular doses are administered to balance the amount of drug being cleared the drug will continue to be active. The time taken to reach the steady state is about five times the half life of a drug. Drugs like digoxin and warfarin with a long half life will take longer to reach a steady state than drugs with a shorter half life.
We can show this using a bucket to represent the body as a container and water to represent the drug. To be effective the drug must reach a certain level and so must our water in the bucket but the body is not a closed system and the drug can be lost by being broken down in the liver and excreted from the kidneys. This loss of drug from the body can be represented by putting a small hole in the bucket so that some water is constantly leaking out. Like the drug level in the body, the level of water drops and needs to be topped up by giving regular doses.
Sometimes a loading dose may be administered so that a steady state is reached more quickly then smaller 'maintenance' doses are given to ensure that the drug levels stay within the steady state.