Metoprolol, sold under the brand name Lopressor among others, is a medication used to treat angina, high blood pressure and a number of conditions involving an abnormally fast heart rate.

[8] The metabolism of metoprolol can vary widely among patients, often as a result of hepatic impairment[9] or CYP2D6 polymorphism.

[25] Patients should be cautious while driving or operating machinery due to its potential to cause decreased alertness.

[27][28] Metoprolol reduces long-term mortality and hospitalisation due to worsening heart failure.

[29] A meta-analysis further supports reduced incidence of heart failure worsening in patients treated with beta-blockers compared to placebo.

[30] However, in some circumstances, particularly when initiating metoprolol in patients with more symptomatic disease, an increased prevalence of hospitalisation and mortality has been reported within the first two months of starting.

[8] Excessive doses of metoprolol can cause bradycardia, hypotension, metabolic acidosis, seizures, and cardiorespiratory arrest.

Blood or plasma concentrations may be measured to confirm a diagnosis of overdose or poisoning in hospitalized patients or to assist in a medicolegal death investigation.

When adrenaline (epinephrine) or noradrenaline (norepinephrine) are released from nerve endings or secreted by the adrenal glands, they bind to β1 adrenergic receptors found primarily in cardiac tissues such as the heart.

[38] Less than 5% of an orally administered dose of metoprolol is excreted unchanged in urine; most of it is eliminated in metabolized form through feces via bile secretion into the intestines.

[38] Metoprolol undergoes extensive metabolism in the liver, mainly α-hydroxylation and O-demethylation through various cytochrome P450 enzymes such as CYP2D6 (primary), CYP3A4, CYP2B6, and CYP2C9.

[11] In the 2000s, a lawsuit was brought against the manufacturers of Toprol XL (a time-release formula version of metoprolol) and its generic equivalent (metoprolol succinate) claiming that to increase profits, lower cost generic versions of Toprol XL were intentionally kept off the market.