ACE inhibitors explained: how they work, side effects and monitoring
6 November 2026 · 2 min read · By the PharmacySkills team
ACE inhibitors are some of the most prescribed medicines in the UK. Ramipril, lisinopril, enalapril and perindopril all belong to the class, and you can spot them by the "-pril" ending. If you understand one mechanism, you can explain every side effect they cause.
How they work
Your kidneys release an enzyme called renin when blood pressure drops. That starts a chain of hormones called the renin–angiotensin–aldosterone system. One step in the chain uses angiotensin-converting enzyme (ACE) to make angiotensin II.
Angiotensin II does two things:
- It narrows blood vessels, which raises blood pressure.
- It triggers the release of aldosterone, which makes the kidneys hold on to salt and water.
An ACE inhibitor blocks that enzyme. Less angiotensin II means wider blood vessels and less fluid retention, so blood pressure falls and the heart has less work to do.
What they are used for
- High blood pressure.
- Heart failure with reduced ejection fraction.
- After a heart attack.
- Kidney disease with protein in the urine, especially in diabetes, where they protect the kidneys.
Side effects, and why they happen
| Problem | Why it happens | What to do |
|---|---|---|
| Dry, tickly cough | ACE also breaks down bradykinin, which builds up | Common. Switch to an angiotensin receptor blocker (ARB) if troublesome |
| Angioedema (swelling of face, lips, tongue or throat) | Bradykinin build-up | Rare but can be life threatening, even after years. Stop and get urgent help. Do not restart |
| Low blood pressure, dizziness | Blood pressure falls, especially with the first dose or with diuretics | Start low; take the first dose at bedtime if at risk |
| High potassium | Less aldosterone means the kidneys keep potassium | Check potassium; care with supplements, potassium-sparing diuretics and salt substitutes |
| Worsening kidney function | Lower pressure in the kidney's filters | Check before starting, 1 to 2 weeks after, and after dose increases. A small creatinine rise is expected |
ACE inhibitors should be avoided in pregnancy, because they can harm the baby's developing kidneys.
Interactions to spot
- NSAIDs reduce the blood pressure effect. Combined with an ACE inhibitor and a diuretic, they can cause acute kidney injury: the so-called "triple whammy".
- Lithium levels can rise to toxic levels.
- Other drugs that raise potassium, such as spironolactone, add to the risk of high potassium.
- Do not combine an ACE inhibitor with an ARB or aliskiren.
Sick day rules
During an illness that causes dehydration, such as vomiting, diarrhoea or fever with sweats, patients should usually pause their ACE inhibitor and restart once they are eating and drinking normally (24 to 48 hours). This protects the kidneys. It is a common counselling point and a common exam question.
Key points to remember
Start low and go slow. Check kidney function and potassium. Warn about the cough and, rarely, angioedema. Avoid in pregnancy. Watch out for NSAIDs.
Sources: BNF: Ramipril; BNF: Angiotensin-converting enzyme inhibitors; NICE guideline NG136: Hypertension in adults; NICE guideline NG106: Chronic heart failure in adults; Think Kidneys: Sick day guidance.
This article is for revision and education. Always check the current BNF, NICE guidance and your local policies before applying it in practice.