Organic Nitrate Pregnancy: C

Isosorbide Mononitrate

Brand names: Ismo, Imdur, Chemydur 60XL

Adult dose

Dose: 20 mg twice daily (standard-release, asymmetric — 8am and 2pm); 40–120 mg once daily (MR)

Route: oral

Frequency: twice daily (standard) or once daily (MR)

Max: 120 mg/day

Asymmetric dosing (8am and 2pm) MANDATORY for standard-release — prevents nitrate tolerance; MR once daily in morning; nitrate-free period of 8h needed

Paediatric dose

Route:

Not licensed in children

Dose adjustments

Renal

No dose adjustment required

Hepatic

No dose adjustment required

Clinical pearls

Nitrate tolerance: ensure nitrate-free period of 8–12h daily — asymmetric dosing essential
Headache: most patients develop tolerance to headache after 2 weeks — use regular paracetamol initially
PDE5 inhibitors (sildenafil, tadalafil, vardenafil): absolute contraindication — potentially fatal hypotension

Contraindications

Hypotension (SBP <90 mmHg)
Hypovolaemia
Severe anaemia
PDE5 inhibitors (sildenafil, tadalafil — fatal hypotension)
Riociguat

Side effects

Headache (most common — dose-dependent)
Flushing
Dizziness
Postural hypotension
Tachycardia
Nitrate tolerance (constant exposure)

Interactions

PDE5 inhibitors (absolute contraindication)
Riociguat (contraindicated)
Antihypertensives (additive hypotension)
Alcohol (enhanced hypotension)

Monitoring

Blood pressure
Headache (tolerance assessment)
Angina frequency

Reference: BNFc; BNF 86; NICE NG185. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Calculators

Kruis Score for Irritable Bowel Syndrome · Functional GI

Drugs

Pathways