Metoprolol succinate ER (Toprol-XL)

Metoprolol is a beta blocker with a preference for the beta1 adrenoceptors of the heart. There it intercepts adrenaline and noradrenaline: heart rate drops, each beat is less forceful, conduction slows and the heart muscle needs less oxygen. At low doses it largely leaves the beta2 receptors in the airways and blood vessels alone, which is why it is called cardioselective. That selectivity is relative and fades as the dose climbs.

ER stands for extended release. Metoprolol tartrate is the immediate-release salt, taken two or three times a day because levels swing widely between doses. Metoprolol succinate in the ER formulation (also labelled CR/XL or ZOK) is made of many small pellets that release the drug evenly over about 24 hours. One tablet a day is enough. This is the exact formulation the large heart failure trials were run with.

In research, metoprolol mostly appears as the reference beta blocker: for blood pressure, heart rate, exercise testing and as the counterweight to anything that drives the sympathetic system. For endurance athletes it is a brake, not a boost, because it caps maximum heart rate. In the range, metoprolol succinate ER comes as an extended-release tablet of 25, 50 or 100mg, 10 tablets per blister.

This information is for research and educational purposes only. No medical advice.

Metoprolol is almost completely absorbed after swallowing but loses about half on its first pass through the liver. The enzyme responsible is CYP2D6. That is the catch: depending on genotype it runs very slowly, normally or ultra-fast. The meta-analysis by Blake and colleagues (Clin Pharmacol Ther 2013) found nearly a fivefold difference in drug exposure between poor and extensive metabolizers. Between poor and ultrarapid ones the difference was thirteenfold. The same tablet can produce very different levels in two people.

The plasma half-life of the active substance is three to seven hours. That one tablet still carries a whole day is down to the formulation: the pellets dissolve gradually in the gut and keep feeding drug throughout the day. Karlson and colleagues (Cardiology 2013) compared the ER form head-to-head with tartrate given several times a day: the same total exposure, but lower peaks, shallower troughs and a more even blockade across the day, visible in heart rate.

Flatter peaks bring a second benefit. Beta1 selectivity is lost at high plasma levels, so clipping the peaks preserves it for longer. The effect on resting heart rate is measurable within hours; the full blood pressure effect takes one to two weeks.

MERIT-HF (Lancet 1999) is the trial that earned the extended-release form its place: 3,991 patients with chronic heart failure, NYHA II to IV and an ejection fraction of 40 percent or less received metoprolol CR/XL or placebo on top of standard therapy. The trial was stopped early because the benefit was too clear to continue: 7.2 versus 11.0 percent deaths per patient-year, a relative risk of 0.66. Sudden deaths fell by 41 percent, deaths from worsening heart failure by 49 percent.

The JAMA analysis (2000) of the same cohort looks at daily life: 19 percent fewer deaths or hospital admissions overall, 31 percent fewer for the composite of death and heart failure admission and 3,401 instead of 5,303 hospital days for heart failure. Both physician-rated NYHA class and patient self-assessment improved.

COMET (Lancet 2003) is the honest counterpoint. Over nearly five years, 34 percent died on carvedilol against 40 percent on metoprolol. The catch: metoprolol there was tartrate at 50mg twice daily, not succinate ER at 200mg. Whether the gap came from the molecule or from dose and formulation is still argued. No such comparison exists for the ER form itself.

Two pharmacokinetic papers complete the picture. Karlson (Cardiology 2013) showed in 27 patients with suspected infarction that 200mg CR/XL once daily delivers the same exposure as tartrate in divided doses, but with lower peaks and a steadier damping of heart rate. Blake (Clin Pharmacol Ther 2013) documents in a meta-analysis of 264 subjects how widely CYP2D6 genotype scatters exposure. Together they explain why the dose is anchored to the pulse, not to the leaflet.

• Fatigue and a performance dip: the most common report, mostly in the first weeks. In training the top end is missing because maximum heart rate is capped. Under endurance load that is measurable.
• Slow pulse and dizziness: resting pulse below 50, greying out on standing, rarely fainting. Combined with verapamil, diltiazem or digoxin this is amplified considerably.
• Cold hands and feet: peripheral vessels tighten a little, clearly noticeable with a Raynaud tendency.
• Sleep and mood: vivid dreams, trouble falling asleep, occasionally low mood.
• Erectile problems: documented in men, usually dose-dependent.
• Airways: in asthma or COPD even a beta1-selective blocker can trigger bronchospasm, especially at higher doses. Anyone who relies on a beta2 inhaler has a real problem here.
• Masked hypoglycaemia: palpitations and tremor as warning signs disappear, sweating remains. Relevant for diabetics.
• Interactions via CYP2D6: paroxetine, fluoxetine, bupropion and quinidine slow the breakdown and raise the level, particularly in poor metabolizers.
• Not suitable with: second- or third-degree AV block, sick sinus syndrome, a resting pulse below 50 even without a blocker, decompensated heart failure and untreated phaeochromocytoma. Anyone on clonidine stops the beta blocker first, otherwise pressure surges when clonidine is withdrawn.
• Withdrawal rebound: a sudden stop can trigger tachycardia, a blood pressure surge and angina. Always taper.

Keep the tablets in their blister, dry, at room temperature and away from light. Not in the bathroom or kitchen, as moisture attacks the extended-release coating. Keep out of reach of children.