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# Dosing regimens and therapeutic drug monitoring

Why some drugs start with a larger first dose, how blood levels guide treatment, and why kidney or liver disease changes dosing.

Updated 2026-11-01

A dosing regimen is the plan of how much and how often. It is built from pharmacokinetics: the volume of distribution sets the loading dose and the clearance sets the maintenance dose. For drugs with a narrow margin, blood levels are measured to keep the patient in range. This page explains the principles in words only; real doses come from the label and local protocols.

## Key points

- Loading dose: fills the volume of distribution quickly; it depends on Vd, not on clearance.
- Maintenance dose: replaces what is eliminated; it depends on clearance.
- Therapeutic drug monitoring measures blood levels of drugs whose effective and harmful levels are close, such as digoxin, aminoglycosides, vancomycin, lithium and some antiseizure medicines.
- Timing matters: peaks and troughs are sampled at set times relative to the dose; a level drawn at the wrong time misleads.
- Kidney impairment: lower the dose, lengthen the interval, or both, for drugs cleared by the kidney. Liver impairment is less predictable.

## Loading and maintenance

When a drug has a long half-life and the effect is needed now, a larger first dose (or several divided doses) fills the body's stores at once; the digoxin label describes this as rapid digitalisation. Maintenance doses then only replace the amount cleared between doses. In kidney failure the loading dose is often unchanged (the volume is similar) while the maintenance dose falls (clearance is lower).

## Peaks, troughs and the dosing interval

The peak is the highest level after a dose; the trough is the lowest, just before the next one. For aminoglycosides such as gentamicin, the label recommends measuring both: a high peak relates to bacterial killing, while a high trough warns of accumulation and toxicity to the kidney and inner ear. Concentration-dependent killing is why aminoglycosides are often given less often at a larger single dose, guided by levels and local protocols.

## Therapeutic drug monitoring

Monitoring is useful when there is a clear link between level and effect, a narrow therapeutic range, large differences between patients, and a reliable assay.

- Antibiotics: amikacin, gentamicin, vancomycin
- Heart: digoxin, amiodarone
- Antiseizure: phenytoin, phenobarbital
- Transplant: ciclosporin, tacrolimus
- Mental health: lithium, valproic acid
- Some drugs are monitored by effect instead of level: warfarin by the INR

## Kidney and liver disease

Kidney function is estimated from creatinine clearance or eGFR, and labels give adjustments for renally cleared drugs (digoxin, aminoglycosides, many antibiotics, enoxaparin and others). Liver disease reduces metabolism and protein synthesis (lower albumin, lower clotting factors), but no single lab value predicts drug clearance, so labels often advise caution or avoidance and careful monitoring.

## Patients who need special care

Newborns, older adults, pregnancy, obesity, critical illness, dialysis and drug interactions all change pharmacokinetics. Weight-based dosing, monitoring of effect and levels, and reading the label for each group are the general approach.

## Memory aid

Load fills the tank (Vd); maintenance tops up the leak (clearance).

## Test yourself

Think of the answer, then tap the question to see it.

## Which pharmacokinetic value sets the size of a loading dose?

The volume of distribution: the loading dose has to fill the space the drug occupies.

## In kidney failure, what usually changes for a renally cleared drug: the loading dose or the maintenance dose?

Mainly the maintenance dose (or the interval), because clearance falls while the volume of distribution is often similar.

## What does a high trough level of gentamicin suggest?

The drug is accumulating between doses, which raises the risk of nephrotoxicity and ototoxicity; the interval or dose needs review.

## What features make a drug a good candidate for therapeutic drug monitoring?

A narrow therapeutic range, a clear link between level and effect, large variation between patients and an assay that is available and reliable.

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## Related classes

- [Aminoglycosides](https://durgical.com/en/pharmacology/classes/aminoglycosides/)
- [Cardiac glycosides](https://durgical.com/en/pharmacology/classes/cardiac-glycosides/)
- [Vitamin K antagonists](https://durgical.com/en/pharmacology/classes/vitamin-k-antagonists/)

## Example drugs

- [Digoxin](https://durgical.com/en/pharmacology/digoxin/)
- [Gentamicin](https://durgical.com/en/pharmacology/gentamicin/)
- [Amikacin](https://durgical.com/en/pharmacology/amikacin/)
- [Vancomycin](https://durgical.com/en/pharmacology/vancomycin/)
- [Warfarin](https://durgical.com/en/pharmacology/warfarin/)
- [Phenytoin](https://durgical.com/en/pharmacology/phenytoin/)
- [Lithium carbonate](https://durgical.com/en/pharmacology/lithium-carbonate/)

[← All of pharmacology](https://durgical.com/en/pharmacology/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Therapeutic Drug Monitoring](https://medlineplus.gov/lab-tests/therapeutic-drug-monitoring/) (MedlinePlus (U.S. National Library of Medicine), 2026); [DailyMed - GENTAMICIN SULFATE injection, solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=977180b3-a222-4282-d485-4a3217674305) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - DIGOXIN tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f3d29508-e7cc-47ff-845d-b5375ee30407) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - WARFARIN SODIUM tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8ad881e0-ca41-42ad-9d7d-eb85b3a30af0) (DailyMed (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/topics/dosing-regimens-and-therapeutic-drug-monitoring/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/topics/dosing-regimens-and-therapeutic-drug-monitoring/