# Centrally Acting Antihypertensives, Alpha-Blockers and Vasodilators

> Methyldopa, clonidine, prazosin, hydralazine and minoxidil for NEET-PG Medicine, covering mechanisms, adverse effects, pregnancy use and classic exam traps.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/centrally-acting-antihypertensives-and-vasodilators
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Katzung's Basic and Clinical Pharmacology; Harrison's Principles of Internal Medicine
- How to cite: "Centrally Acting Antihypertensives, Alpha-Blockers and Vasodilators", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/centrally-acting-antihypertensives-and-vasodilators

## Direct answer

Centrally acting antihypertensives such as methyldopa and clonidine lower blood pressure by reducing sympathetic outflow from the brainstem. Prazosin blocks postsynaptic alpha-1 receptors, while hydralazine and minoxidil relax arterioles directly. Methyldopa is a long-established choice for hypertension in pregnancy, and clonidine must never be stopped abruptly because of rebound hypertension.

## What you must remember

- **Methyldopa:** a prodrug converted to alpha-methylnorepinephrine that acts on central alpha-2 receptors; adverse effects include a positive direct Coombs test, haemolytic anaemia and drug-induced hepatitis.
- **Clonidine:** a central alpha-2 agonist that causes dry mouth, sedation and bradycardia; abrupt withdrawal produces rebound hypertension.
- **Prazosin:** a selective alpha-1 blocker; the first dose can cause marked postural hypotension and syncope, and reflex tachycardia is minimal.
- **Phenoxybenzamine:** an irreversible, non-selective alpha blocker used before surgery for phaeochromocytoma.
- **Hydralazine:** a direct arteriolar vasodilator that causes reflex tachycardia and sodium retention, so it is given with a beta blocker and a diuretic.
- **Drug-induced lupus:** a lupus-like syndrome seen with hydralazine, more often in slow acetylators.
- **Minoxidil:** a potassium channel opener that causes hypertrichosis, pericardial effusion and fluid retention; topical use treats alopecia.

## Choosing an agent by site of action and adverse effect

Each drug in this group acts at a different point in the sympathetic or vascular pathway, and that site explains most of its adverse effects. The table groups the agents by where they act and the adverse effect most likely to be asked.

| Drug | Site of action | Signature adverse effect |
|---|---|---|
| Methyldopa | Central alpha-2 receptors | Haemolytic anaemia and positive Coombs test |
| Clonidine | Central alpha-2 receptors | Rebound hypertension on withdrawal |
| Prazosin | Postsynaptic alpha-1 receptors | First-dose syncope |
| Phenoxybenzamine | Alpha-1 and alpha-2 receptors, irreversibly | Postural hypotension and reflex tachycardia |
| Hydralazine | Arteriolar smooth muscle | Reflex tachycardia and lupus-like syndrome |
| Minoxidil | Potassium channels in arterioles | Hypertrichosis and pericardial effusion |

Central agents act in the brainstem, so their problems are sedation, dry mouth and withdrawal effects that show up over weeks of use. Direct vasodilators and non-selective alpha blockers instead trigger baroreflex responses such as tachycardia and fluid retention, which is why they are usually combined with a beta blocker or a diuretic in clinical practice and in exam answers.

## Traps that cost marks

These traps usually turn on a drug's adverse effect or a safety rule rather than on its mechanism.

- **Trap:** choosing clonidine as the default antihypertensive in pregnancy because it is centrally acting — **Correct:** methyldopa is the long-established choice in pregnancy, and it is the answer the exam expects.
- **Trap:** stopping clonidine abruptly in a patient who has taken it for months — **Correct:** rebound hypertension follows abrupt withdrawal, so the dose is tapered or other agents are introduced first.
- **Trap:** giving the full dose of prazosin on the first day — **Correct:** first-dose syncope is the classic problem, so the first dose is low and taken at bedtime.
- **Trap:** using hydralazine alone for chronic hypertension — **Correct:** reflex tachycardia and sodium retention make combination with a beta blocker and a diuretic necessary.

## Frequently asked questions

### Why is methyldopa preferred for hypertension in pregnancy?

Methyldopa has a long record of use in pregnancy, which is why it is the classic answer in NEET-PG questions on this topic. Current guidance lists several agents for chronic hypertension in pregnancy, so the best answer depends on the exact wording of the question. Its adverse effects, such as haemolytic anaemia, still need monitoring in methyldopa users.

### Which adverse effect of methyldopa is tested most often?

The classic answer is a positive direct Coombs test with haemolytic anaemia, caused by autoantibodies against red cells that methyldopa induces. Drug-induced hepatitis is the other important adverse effect of methyldopa. Stopping methyldopa is the first step when either appears.

### Why must clonidine not be stopped suddenly?

Abrupt withdrawal of clonidine causes rebound hypertension, sometimes with agitation, tachycardia and sweating. This happens because the central sympathetic outflow that clonidine suppressed recovers quickly. The dose is tapered over several days, or other antihypertensives are introduced first.

### Why is a beta blocker given along with hydralazine?

Hydralazine causes reflex tachycardia and sodium and water retention, which can worsen angina and heart failure. A beta blocker blunts the reflex tachycardia, and a diuretic counters the fluid retention. This combination is the standard way hydralazine is used for chronic hypertension.

### What is the first-dose effect of prazosin?

The first-dose effect is a sudden fall in blood pressure on standing, which can cause syncope soon after the first dose. It occurs because alpha-1 blockade dilates vessels without a compensatory rise in heart rate. Starting at a low dose at bedtime reduces the risk of prazosin-induced syncope.