The steady state is the condition when the rate of drug administration is equal to the rate of drug elimination.
When the drug is administered as a constant rate IV infusion, there will be one steady state concentration as long as the rate of administration and the rate of elimination do not change.
When drug administration involves repeated doses, the drug concentration-time profile during each of the dosing intervals ( τ ) will be identical at steady state. This means that the maximum, the minimum, and the average concentrations during each dosing interval will be similar at steady state.
The therapeutic range is defined as the range of plasma drug concentration in which the probability of the desired therapeutic effect is high and the probability of the unacceptable toxicity is low. The lower limit of the therapeutic range is the minimum effective concentration (MEC) and the upper limit of the therapeutic range is the minimum toxic concentration (MTC).
The therapeutic range represents a range of drug conc in plasma and not at the site of action. This means that when the plasma conc of the drug is within the therapeutic range, the drug conc at the site of action (may be lower or higher than the plasma conc) produces the desired therapeutic effect.
The therapeutic range is different for different drugs. When the therapeutic range is wide, the drug is said to have a large therapeutic index, while when the therapeutic range is narrow, it is said that the drug has small therapeutic index. Drugs with small therapeutic index require careful selection of the dosing regimen to achieve therapeutic drug concentration.
The dosing interval is the period of time between administration of doses.
The choice of dosing interval can help the patient to remember the time for drug administration. Dosing intervals in which the time for drug administration is the same every day are much better than when the time of drug administration changes every day. For example:
Drugs given every 6 hr (6:00 AM, 12:00 AM, 6:00 PM and 12:00 PM)
Drugs given every 8 hr (8:00 AM, 4:00 PM, and 12:00PM)
Drugs given every 12 hr (8:00 AM, and 8:00 PM)
Practical dosing intervals are : 6 hr, 8 hr, 12 hr, 18 hr, 24 hr, 48 hr, 72 hr etc.