How Blood Pressure Medications Work. A Plain-English Guide to the Drug Classes.

There are four main classes of antihypertensive medications, each working through a different mechanism on the cardiovascular system. Understanding how they work explains why drug selection matters, why switching classes can solve side effects that switching doses cannot, and why combination therapy produces additive rather than duplicative effects. The man who understands his antihypertensive regimen is more likely to stay on it and more likely to report problems that can be fixed.

The Mechanism

The renin-angiotensin-aldosterone system and the drugs that target it

Blood pressure is regulated by a hormonal cascade called the renin-angiotensin-aldosterone system, or RAAS. The kidney releases renin when it detects low blood pressure or low sodium. Renin cleaves angiotensinogen (produced by the liver) to angiotensin I. Angiotensin-converting enzyme (ACE), located primarily in lung endothelium, converts angiotensin I to angiotensin II. Angiotensin II is a potent vasoconstrictor that also stimulates the adrenal gland to release aldosterone, which drives sodium and water retention in the kidney.

The net effect of RAAS activation: blood vessels constrict, the kidneys retain salt and water, blood volume rises, and blood pressure increases. This is appropriate when blood pressure is genuinely low. In hypertension, the system is activated or dysregulated in ways that sustain chronically elevated pressure.

ACE inhibitors (lisinopril, enalapril, ramipril, perindopril, benazepril) block the conversion of angiotensin I to angiotensin II. With less angiotensin II in circulation, the arteries relax, aldosterone release falls, and the kidneys retain less sodium. Blood pressure drops through both vasodilation and volume reduction.

The same ACE enzyme that converts angiotensin I also degrades bradykinin, a vasodilatory peptide. When ACE is inhibited, bradykinin accumulates. Elevated bradykinin has beneficial vasodilatory effects, which may contribute to the efficacy of ACE inhibitors in heart failure and post-MI settings. But in roughly 5 to 15 percent of patients, bradykinin accumulation also causes a dry, persistent cough from airway irritation. This cough is not dangerous, but it is the most common reason men stop ACE inhibitors without telling their physician.

ARBs (angiotensin receptor blockers: losartan, valsartan, irbesartan, olmesartan, telmisartan) do not block ACE. They block the receptor that angiotensin II binds to, the AT1 receptor, preventing angiotensin II from exerting its vasoconstrictive and sodium-retaining effects. Because ARBs leave the ACE enzyme untouched, bradykinin is not elevated and the cough does not occur. Blood pressure reduction is comparable to ACE inhibitors. The side effect profile is generally better tolerated.

Both ACE inhibitors and ARBs are preferred in specific clinical contexts: diabetic nephropathy (where they reduce proteinuria and slow renal disease progression through intraglomerular pressure reduction), heart failure with reduced ejection fraction (where mortality benefit is documented in multiple randomized trials), and post-myocardial infarction management. They are less effective as initial monotherapy in Black patients with hypertension, where the low-renin phenotype means the RAAS is less the dominant driver of elevated blood pressure and calcium channel blockers or thiazides typically produce better initial response.

Calcium channel blockers and the arterial wall

Calcium channel blockers (CCBs) work at a different point in the vascular physiology. Smooth muscle contraction in arterial walls requires calcium entry through voltage-gated L-type calcium channels. When calcium enters, the smooth muscle contracts and the artery narrows. CCBs block these channels, preventing calcium entry, reducing smooth muscle tone, and relaxing the arterial wall. The result is vasodilation and lower blood pressure.

Dihydropyridine CCBs (amlodipine, nifedipine, felodipine) act primarily on peripheral vascular smooth muscle with minimal effect on cardiac conduction. Amlodipine has the longest half-life in the class, approximately 30 to 50 hours, which makes once-daily dosing effective and provides protection against the blood pressure surge that occurs with missed doses. This class is effective across all populations and phenotypes and is the preferred first-line agent in Black patients with hypertension.

Non-dihydropyridine CCBs (diltiazem, verapamil) act on both peripheral arteries and cardiac cells. They slow the heart rate and atrioventricular node conduction, making them useful in rate control for atrial fibrillation and in some arrhythmias. However, this cardiac effect also means they can worsen heart failure with reduced ejection fraction by depressing cardiac contractility. They require caution in patients with systolic dysfunction.

The most common side effect of dihydropyridine CCBs is ankle edema from local vasodilation in the lower extremities, which is not a sign of fluid overload but of increased hydrostatic pressure in dilated capillaries. Constipation occurs with verapamil in particular, due to calcium channel effects on intestinal smooth muscle.

What the Evidence Shows

Thiazide diuretics: the cardiovascular outcomes data

Thiazide diuretics (hydrochlorothiazide, chlorthalidone, indapamide) work by blocking the sodium-chloride cotransporter in the distal convoluted tubule of the nephron, reducing sodium reabsorption. More sodium in the tubular fluid draws water with it through osmosis. The result is increased urine output, reduced plasma volume, reduced cardiac preload, and lower blood pressure. The initial blood pressure effect is volume-related. With long-term use, the predominant mechanism shifts toward persistent vasodilation through mechanisms that are not fully characterized.

The ALLHAT trial (Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial), published by the ALLHAT Officers in JAMA in 2002, enrolled 33,357 hypertensive adults aged 55 or older at high cardiovascular risk. Participants were randomized to chlorthalidone, amlodipine, or lisinopril. Chlorthalidone was equivalent or superior to the other two agents for the primary outcome of fatal coronary heart disease or nonfatal MI. The chlorthalidone arm also had significantly lower rates of heart failure than the amlodipine or lisinopril arms. ALLHAT established chlorthalidone’s place as a first-line antihypertensive backed by hard outcomes data, not merely blood pressure reduction.

Chlorthalidone differs from hydrochlorothiazide in clinically important ways. Chlorthalidone has a longer effective half-life (approximately 45 to 60 hours vs. 9 to 13 hours for HCTZ), greater blood pressure lowering at equivalent doses, and superior cardiovascular outcomes evidence. Current guidelines from the American College of Cardiology and the American Heart Association recommend chlorthalidone as the preferred thiazide-type diuretic when a diuretic is chosen.

Side effects of thiazide diuretics include hypokalemia (low potassium), which can cause muscle cramps and, in severe cases, cardiac arrhythmias. Potassium monitoring is standard with thiazide use, and many patients require either dietary potassium supplementation or a potassium-sparing diuretic added to the regimen. Thiazides also raise uric acid levels (increasing gout risk in susceptible individuals) and modestly impair glucose metabolism, a relevant consideration in patients with prediabetes or metabolic syndrome.

Beta-blockers: mechanism and appropriate use

Beta-blockers (metoprolol succinate, carvedilol, bisoprolol, atenolol, nebivolol) block beta-adrenergic receptors on the heart and elsewhere, reducing both heart rate and the force of cardiac contraction. Cardiac output (the product of heart rate and stroke volume) falls, and blood pressure drops. Some beta-blockers (carvedilol, nebivolol) also have peripheral vasodilatory properties that contribute to blood pressure reduction.

Current ACC/AHA guidelines do not recommend beta-blockers as first-line therapy for uncomplicated hypertension, meaning hypertension without comorbidities that specifically benefit from beta-blockade. The evidence for their superiority to other classes in reducing cardiovascular events in uncomplicated hypertension is weaker than for the other three classes. However, they remain first-line and have documented mortality benefit in specific situations: heart failure with reduced ejection fraction (the landmark MERIT-HF and COPERNICUS trials showed 34 to 35 percent mortality reduction with metoprolol succinate and carvedilol respectively), post-myocardial infarction management, and rate control in atrial fibrillation.

The side effects most relevant to men: fatigue and reduced exercise capacity from blunted heart rate response to exertion, and erectile dysfunction. Older, non-selective beta-blockers such as atenolol and propranolol carry more risk of erectile dysfunction due to peripheral vasoconstriction and blunted sympathetic response to arousal. Newer vasodilating beta-blockers, particularly nebivolol, produce less sexual dysfunction through nitric oxide-mediated vasodilation that partially offsets the vasoconstrictive tendency of beta-receptor blockade. A man who stops his beta-blocker because of fatigue or erectile dysfunction and does not mention it to his physician is at risk of undertreated hypertension and the cardiovascular sequelae that follow.

Combination Therapy and the Adherence Advantage

The most consistent obstacle to blood pressure control in clinical practice is not medication efficacy — it is adherence. Studies measuring prescription fill rates and pharmacy refill patterns consistently find that a substantial proportion of patients on antihypertensive therapy are taking their medication intermittently, reducing doses, or stopping without informing their clinician. The gap between blood pressure control that is achievable and blood pressure control that is achieved is primarily an adherence gap.

The number of medications in a daily regimen is one of the strongest predictors of adherence. Research on polypharmacy in hypertension has consistently found that adherence drops meaningfully with each additional pill. This creates a specific clinical problem: the patients who need combination therapy to reach blood pressure targets — which is a majority of hypertensive patients, given that monotherapy controls blood pressure in only about half of cases — are the same patients whose adherence is most likely to deteriorate with a multi-drug regimen.

Fixed-dose combination tablets address this by compressing two or three medications into a single pill taken once daily. The available combinations cover the mechanistically rational pairings: amlodipine with perindopril, amlodipine with valsartan, telmisartan with amlodipine, and three-drug combinations incorporating a calcium channel blocker, a RAAS blocker, and hydrochlorothiazide. Because patients take one pill rather than three, adherence rates are meaningfully higher, and blood pressure control is maintained more consistently across the intervals between doses.

The ACCOMPLISH trial (Jamerson and colleagues, New England Journal of Medicine, 2008) tested two fixed-dose combination strategies in 11,506 patients at high cardiovascular risk: benazepril plus amlodipine versus benazepril plus hydrochlorothiazide. Despite very similar blood pressure reductions between the two groups, the amlodipine-based combination reduced the primary composite of cardiovascular death, MI, and stroke by 20 percent compared to the HCTZ-based combination. The trial demonstrated that the choice of which mechanisms to combine matters beyond the blood pressure reduction achieved — mechanistically complementary pairings produce outcomes that blood pressure numbers alone do not fully explain.

The 2018 ESC/ESH hypertension guidelines shifted position on when to initiate combination therapy, recommending that most patients begin with a two-drug combination rather than monotherapy titrated to maximum dose. The rationale: adding a second complementary mechanism produces greater blood pressure reduction than increasing monotherapy dose, does so with fewer side effects because both drugs operate at lower individual doses, and typically achieves target faster. For patients currently on monotherapy at maximum dose with blood pressure still above target, the evidence-based question is whether a lower-dose combination of two complementary agents would be both more effective and better tolerated.

What to Do This Week

1. Know what class of medication you are on and what mechanism it targets. This is printed on your medication label or available from your pharmacist in 30 seconds. Knowing whether you are on an ACE inhibitor, an ARB, a CCB, a thiazide, or a beta-blocker tells you what your physician thought was driving your hypertension and what side effects to watch for.

2. If you have a persistent dry cough and are on an ACE inhibitor, mention it to your physician at your next visit. Switching from an ACE inhibitor to an ARB eliminates this side effect without changing the mechanism of blood pressure control. ACE inhibitor cough resolves within 1 to 4 weeks of switching. Do not stop the medication without a replacement in place.

3. If you are on a beta-blocker and experiencing fatigue or erectile dysfunction, ask about your options. If you are on a beta-blocker for hypertension alone (without heart failure or arrhythmia), your physician may be able to switch to a different class. If you need to remain on a beta-blocker, nebivolol has the most favorable sexual side effect profile in the class.

4. If your blood pressure is not at target on your current medication, the right conversation is about mechanism, not just dose. Ask specifically: which mechanism are we using, have we reached the maximum dose of this mechanism, and is adding a second drug with a complementary mechanism (such as a CCB added to an ACE inhibitor) the appropriate next step? Doubling a dose rarely produces double the blood pressure reduction. Combining two mechanistically distinct drugs often does.

5. If you are on hydrochlorothiazide, ask your physician whether chlorthalidone would be appropriate. Chlorthalidone has superior outcomes evidence and longer duration of action. Switching is simple and may improve blood pressure control without adding a new drug.

Blood pressure medication works through specific, understandable physiology. The physician who explains the mechanism is giving you the information you need to report side effects, understand the rationale for drug selection, and make the decisions that keep blood pressure controlled over the long term. That conversation is worth asking for.