PSA · Drug monitoring reference

Drug monitoring

The high-yield drugs whose levels, efficacy and toxicity you must monitor — with therapeutic ranges, timing, and what pushes them into the danger zone. Tap the dots to track what you know.0/27 rated

Drug levels

LithiumNTI

Mood stabiliser

Lithium toxicity — narrow therapeutic index

0.4–1 mmol/L· 12 hours post-dose

Monitor

serum lithium level
U&E / renal function
thyroid function (TFTs)
calcium

Baseline

U&E / renal function
TFTs
calcium
weight
ECG (if cardiac risk)

Toxicity

coarse tremor
vomiting & diarrhoea
confusion
ataxia / slurred speech
seizures

Raised by

NSAIDs
ACE inhibitors / ARBs
thiazide diuretics
dehydration

level every 3 months once stable; weekly after a dose change; U&E/TFT every 6 monthsBNF — lithium (TDM)

DigoxinNTI

Cardiac glycoside

Digoxin toxicity — worse with hypokalaemia

0.8–2 micrograms/L· ≥6 hours post-dose

Monitor

serum potassium / U&E
renal function
heart rate

Baseline

U&E / potassium
renal function

Toxicity

nausea & vomiting
confusion
xanthopsia (yellow-green vision)
bradycardia / arrhythmia

Raised by

hypokalaemia
amiodarone
verapamil
diuretics (via low K⁺)

level not routine — check if toxicity suspected or poor response; monitor K⁺/renal functionBNF — digoxin (TDM)

GentamicinNTI

Aminoglycoside antibiotic

Nephrotoxicity & ototoxicity

Individualised level· pre-dose (trough) ± post-dose (peak)

Monitor

serum gentamicin (peak & trough)
renal function

Baseline

renal function
weight

Toxicity

nephrotoxicity (acute kidney injury)
ototoxicity (hearing loss, vertigo)

Raised by

renal impairment
loop diuretics
vancomycin

trough before the 3rd–4th dose; renal function regularlyBNF — aminoglycosides (TDM)

VancomycinNTI

Glycopeptide antibiotic

Nephrotoxicity

10–20 mg/L· pre-dose (trough), from day 2

Monitor

serum vancomycin (trough)
renal function

Baseline

renal function
weight

Toxicity

nephrotoxicity
"red man" syndrome (rapid infusion)
ototoxicity

Raised by

renal impairment
aminoglycosides

trough levels guide dosing; renal function regularlyBNF — vancomycin (TDM)

Theophylline / aminophyllineNTI

Methylxanthine bronchodilator

Toxicity — the toxic dose is close to the therapeutic dose

10–20 mg/L· 4–6 h after an oral dose (5 days after starting)

Monitor

theophylline level
potassium
heart rate

Baseline

baseline level if switching from oral

Toxicity

tachycardia / arrhythmia
vomiting
tremor
agitation
hypokalaemia
seizures

Raised by

macrolides
ciprofloxacin
cimetidine
heart failure (lowered by smoking)

level 5 days after starting and after any dose changeBNF — theophylline (TDM)

PhenytoinNTI

Antiepileptic

Phenytoin toxicity — zero-order kinetics (small dose change, big level change)

10–20 mg/L· trough (pre-dose), at steady state

Monitor

phenytoin level
FBC
clinical features

Baseline

FBC
LFTs

Toxicity

nystagmus
ataxia
slurred speech
diplopia
confusion

Raised by

hepatic impairment
many enzyme interactions
hypoalbuminaemia (free level)

level at steady state, and if toxicity or poor seizure controlBNF — phenytoin (TDM)

Anticoagulation

WarfarinNTI

Vitamin K antagonist

Over-anticoagulation / bleeding (and under-treatment)

2–3 INR· —

Monitor

Baseline

baseline INR
LFTs
FBC

Toxicity

bleeding / bruising
haematuria
GI bleed

Raised by

macrolides / metronidazole / ciprofloxacin
amiodarone
NSAIDs (bleed risk)
cranberry juice
acute alcohol

INR frequently until stable, then up to every 12 weeksBNF — warfarin (target INR 2–3; 3–4 for recurrent VTE / mechanical valves)

Unfractionated heparin (IV)

Anticoagulant

Bleeding & heparin-induced thrombocytopenia (HIT)

Monitor

APTT
platelet count
potassium

Baseline

APTT
FBC / platelets
U&E

Toxicity

bleeding
HIT (falling platelets, day 5–10)
hyperkalaemia

Raised by

renal impairment
other antithrombotics

APTT 6-hourly (adjust infusion); platelets if treatment >4 daysBNF — heparin (APTT-adjusted)

Low molecular weight heparin

Anticoagulant

Bleeding & hyperkalaemia — but usually NO routine monitoring

Monitor

no routine monitoring
anti-Xa (only special cases)
platelets
renal function
potassium

Baseline

FBC / platelets
renal function
weight

Toxicity

bleeding
hyperkalaemia
HIT (less than UFH)

Raised by

renal impairment (accumulates)

no routine monitoring; anti-Xa only in pregnancy, renal impairment, or extremes of weightBNF — LMWH

DOAC (apixaban / rivaroxaban)

Direct oral anticoagulant

Bleeding — but NO routine coagulation monitoring needed

Monitor

no routine coagulation monitoring
renal function
FBC

Baseline

renal function
FBC
LFTs

Toxicity

bleeding

Raised by

renal impairment
strong CYP3A4 / P-gp inhibitors

renal function at least annually (more often if CKD / elderly); no routine INR or anti-XaBNF — DOACs

DMARDs & cytotoxics

MethotrexateNTI

DMARD / antifolate (WEEKLY)

Bone marrow suppression, hepatotoxicity & pneumonitis — dosed WEEKLY

Monitor

FBC
LFTs
U&E / renal function

Baseline

FBC
U&E / renal function
LFTs
chest X-ray

Toxicity

bone marrow suppression
hepatotoxicity
pneumonitis (dry cough, breathlessness)
mucositis / stomatitis

Raised by

trimethoprim / co-trimoxazole
NSAIDs
renal impairment

FBC, U&E & LFTs every 1–2 weeks until stable, then every 2–3 monthsBNF — methotrexate (MHRA: weekly dosing)

AzathioprineNTI

Immunosuppressant / thiopurine

Myelosuppression — check TPMT activity first

Monitor

FBC
LFTs

Baseline

TPMT activity
FBC
LFTs

Toxicity

bone marrow suppression
hepatotoxicity
pancreatitis

Raised by

allopurinol (blocks metabolism — reduce dose)
low TPMT activity

FBC weekly initially (then less often, e.g. 3-monthly); LFTsBNF — azathioprine

Amiodarone

Antiarrhythmic

Thyroid, liver, lung & eye toxicity (long half-life)

Monitor

thyroid function (TFTs)
liver function (LFTs)

Baseline

TFTs
LFTs
U&E / potassium
chest X-ray
ECG

Toxicity

hypo- or hyper-thyroidism
hepatotoxicity
pulmonary fibrosis
corneal microdeposits
slate-grey skin / photosensitivity

Raised by

raises digoxin & warfarin levels
many CYP interactions

TFTs and LFTs every 6 monthsBNF — amiodarone

CiclosporinNTI

Calcineurin inhibitor

Nephrotoxicity & hypertension

Individualised level· pre-dose (trough)

Monitor

ciclosporin level (trough)
renal function
blood pressure
potassium
LFTs

Baseline

renal function
blood pressure
LFTs
potassium
lipids

Toxicity

nephrotoxicity
hypertension
gum hyperplasia
hirsutism
tremor

Raised by

grapefruit juice / CYP3A4 inhibitors
nephrotoxic drugs

trough levels, renal function and BP regularlyBNF — ciclosporin

Psychiatry

Clozapine

Atypical antipsychotic

Agranulocytosis (neutropenia) & myocarditis

Monitor

FBC (white cells / neutrophils)
weight
fasting glucose & lipids
ECG
troponin/CRP (myocarditis)

Baseline

FBC
ECG
weight
lipids & glucose

Toxicity

agranulocytosis (neutropenia)
myocarditis
constipation / ileus
seizures
hypersalivation

Raised by

smoking cessation raises levels
caffeine

FBC weekly for 18 weeks, then fortnightly to 1 year, then monthlyBNF — clozapine (stop if neutrophils <1.5×10⁹/L)

Antipsychotics (general)

Antipsychotic

Metabolic syndrome & QT prolongation

Monitor

weight / BMI
fasting glucose / HbA1c
lipids
prolactin
ECG (QT)
blood pressure

Baseline

weight
glucose
lipids
ECG
prolactin

Toxicity

weight gain / diabetes
QT prolongation
extrapyramidal effects
hyperprolactinaemia

weight frequently in the first year; glucose & lipids at 12 weeks then annuallyBNF — antipsychotics

Sodium valproate

Antiepileptic / mood stabiliser

Hepatotoxicity (esp. first 6 months); highly teratogenic

Monitor

LFTs
FBC

Baseline

LFTs
FBC
weight
pregnancy status / contraception

Toxicity

hepatotoxicity
pancreatitis
thrombocytopenia
weight gain
tremor

Raised by

hepatic impairment

LFTs especially during the first 6 months; level is not routinely usefulBNF — valproate (Pregnancy Prevention Programme)

Endocrine & CV

Statins

Lipid-lowering

Myopathy / rhabdomyolysis & hepatotoxicity

Monitor

LFTs
creatine kinase (only if muscle symptoms)
lipids / non-HDL

Baseline

LFTs
lipids
HbA1c
renal function
TFTs

Toxicity

myalgia / myopathy
rhabdomyolysis (raised CK)
deranged LFTs

Raised by

macrolides
some calcium-channel blockers
grapefruit juice

LFTs at baseline, 3 months and 12 months; CK only if muscle painBNF — statins

ACE inhibitors

Antihypertensive / RAAS

Hyperkalaemia & renal impairment

Monitor

U&E
creatinine / eGFR
potassium

Baseline

U&E
eGFR
potassium

Toxicity

hyperkalaemia
acute kidney injury / rising creatinine
hypotension (first dose)
dry cough / angioedema

Raised by

potassium-sparing diuretics / K⁺ supplements
NSAIDs
renal artery stenosis

U&E before starting, 1–2 weeks after starting / each dose increase, then periodicallyBNF — ACE inhibitors (allow ≤30% creatinine rise / K⁺ ≤5.5)

Insulin / diabetes

Antidiabetic

Hypoglycaemia; glycaemic control

Monitor

capillary blood glucose
HbA1c

Baseline

HbA1c
weight
renal function

Toxicity

hypoglycaemia
weight gain

Raised by

missed meals / alcohol
renal impairment (insulin accumulates)

HbA1c every 3–6 months; daily capillary glucose self-monitoringBNF — insulin (HbA1c target 48–53 mmol/mol)

Carbimazole

Antithyroid

Agranulocytosis — report sore throat / fever

Monitor

FBC (urgently if signs of infection)
TFTs

Baseline

FBC
LFTs
TFTs

Toxicity

agranulocytosis (sore throat, fever, mouth ulcers)
hepatotoxicity

TFTs to titrate the dose; FBC only if clinical signs of infection (not routine)BNF — carbimazole (stop if neutropenia)

Levothyroxine

Thyroid hormone

Over- or under-replacement (titrate to TSH)

Monitor

TSH
free T4 (if needed)

Baseline

TFTs (TSH, free T4)

Toxicity

over-replacement → atrial fibrillation, osteoporosis, palpitations, tremor

Raised by

iron / calcium / PPIs reduce absorption (separate doses)

TSH 8–12 weeks after starting / dose change, then annually once stableBNF — levothyroxine

Spironolactone

Aldosterone antagonist / K⁺-sparing diuretic

Hyperkalaemia

Monitor

U&E
potassium
renal function

Baseline

U&E
potassium

Toxicity

hyperkalaemia
acute kidney injury
gynaecomastia

Raised by

ACE inhibitors / ARBs
NSAIDs
K⁺ supplements

U&E within 1 week of starting / dose change, then periodicallyBNF — spironolactone

Other safety

Oxygen

Medical gas

Hypoxia vs CO₂ retention — target by patient

Monitor

oxygen saturation (SpO₂)
arterial blood gas (if CO₂ retention risk)

Baseline

SpO₂
ABG if unwell / COPD

Toxicity

CO₂ retention / narcosis (in COPD)

Raised by

uncontrolled high-flow O₂ in COPD

continuous SpO₂; ABG if at risk of hypercapniaBNF — oxygen (target 94–98%; 88–92% in COPD)

Blood transfusion

Blood product

Transfusion reaction & fluid overload (TACO)

Monitor

observations (temp, pulse, BP, RR)
post-transfusion FBC

Baseline

group & save / crossmatch
baseline observations

Toxicity

acute transfusion reaction (fever, rigors)
TACO (fluid overload)
haemolytic reaction

Raised by

rapid transfusion in cardiac failure (TACO)

observations before, at 15 minutes, during and after each unitBNF / BSH — transfusion

IV fluids

Fluid therapy

Electrolyte disturbance & fluid overload

Monitor

U&E (daily)
fluid balance
weight

Baseline

U&E
weight
clinical fluid status

Toxicity

fluid overload (pulmonary oedema)
hyponatraemia
hyperkalaemia

Raised by

excess 5% glucose → hyponatraemia
heart / renal failure

U&E and fluid balance reviewed dailyBNF — fluids and electrolytes

Combined oral contraceptive

Hormonal contraceptive

Blood pressure (and VTE / migraine risk)

Monitor

blood pressure
weight / BMI

Baseline

blood pressure
BMI
migraine history
VTE risk

Toxicity

venous thromboembolism
hypertension
stroke (esp. migraine with aura)

Raised by

smoking + age >35
enzyme inducers reduce efficacy

BP and weight at baseline, at 3 months, then annuallyBNF — combined hormonal contraceptives