Amlodipine Travel & Timezone-Shift Protocols: A Clinical Guide

What the Norvasc label says about dose timing

Norvasc (amlodipine besylate) is a dihydropyridine calcium channel blocker taken once daily. For hypertension the usual adult starting dose is 5 mg once daily, the maximum is 10 mg once daily, and the label advises waiting 7 to 14 days between titration steps unless faster titration is clinically warranted [1]. Peak plasma concentration arrives 6 to 12 hours after a dose, steady state is reached after 7 to 8 days of daily dosing, and the terminal elimination half-life is about 30 to 50 hours [1].

Absolute bioavailability is 64% to 90% and is not altered by food [1]. With chronic once-daily oral dosing, the label states that antihypertensive effectiveness is maintained for at least 24 hours [1].

The labeled indications are hypertension, chronic stable angina, vasospastic (Prinzmetal) angina, and angiographically documented coronary artery disease in patients without heart failure or an ejection fraction below 40% [1].

Onset, half-life and hypotension

Amlodipine has a gradual onset of effect, because its interaction with the calcium channel receptor has a gradual rate of association and dissociation [1]. The label says acute hypotension is unlikely for this reason, though symptomatic hypotension is possible, particularly in severe aortic stenosis [1].

Overdose

The label describes overdose as expected to cause excessive peripheral vasodilation with marked hypotension and possibly reflex tachycardia. Hemodialysis is unlikely to help because amlodipine is highly protein bound (about 93%) [1].

Travel antibiotics and other medicines that raise amlodipine exposure

The label warns that moderate and strong CYP3A inhibitors increase systemic exposure to amlodipine and may require a dose reduction, with monitoring for hypotension and edema [1]. Diltiazem 180 mg daily given with amlodipine 5 mg to elderly hypertensive patients raised amlodipine exposure by 60% [1]. Erythromycin did not significantly change exposure in healthy volunteers, but strong inhibitors such as itraconazole and clarithromycin may raise concentrations more [1].

In a population-based cohort from Ontario, adults with a mean age of 76 who were already taking a calcium-channel blocker (amlodipine in more than half) were newly prescribed clarithromycin (n=96,226) or azithromycin (n=94,083). Within 30 days, in the clarithromycin versus azithromycin groups, hospitalization with acute kidney injury occurred in 0.44% versus 0.22% (odds ratio 1.98, 95% CI 1.68-2.34), hospitalization with hypotension in 0.12% versus 0.07% (odds ratio 1.60), and all-cause mortality in 1.02% versus 0.59% (odds ratio 1.74) [2].

The other direction matters too. The label says no quantitative information is available on CYP3A inducers, and that blood pressure should be closely monitored when amlodipine is given with one [1]. Amlodipine is itself a weak CYP3A inhibitor, and the label caps simvastatin at 20 mg daily in patients taking it [1][3]. The label also says to monitor for hypotension when sildenafil and amlodipine are used together; each independently lowers blood pressure [1].

Grapefruit juice, food and meal timing

The Norvasc label states that grapefruit juice has no impact on amlodipine exposure, listing it alongside cimetidine and magnesium and aluminum hydroxide antacids [1]. Two small crossover studies fit that description. In 12 healthy men, a single 5 mg dose with 250 mL of grapefruit juice produced a Cmax of 115% and an AUC over 72 hours of 116% of the water values, with no significant difference in blood pressure or heart rate [4]. In 20 healthy men given a single 10 mg amlodipine dose, grapefruit juice or water was given once daily for 9 days; oral availability was 88% with juice and 81% with water, and AUC, Cmax and clearance were not markedly changed [5]. This differs from simvastatin, where the simvastatin label says to avoid grapefruit juice [3].

Food does not change amlodipine absorption: bioavailability is 64% to 90% and is not altered by food [1].

Side effects that stand out on a trip

Edema is the most common adverse reaction and is dose related. In placebo-controlled trials, edema occurred in 1.8% of patients on 2.5 mg, 3.0% on 5 mg and 10.8% on 10 mg, versus 0.6% on placebo [1]. It was more frequent in women (14.6% on amlodipine versus 5.1% on placebo) than in men (5.6% versus 1.4% on placebo) [1].

Liver, kidney, age and who has less slack

Renal impairment does not significantly change amlodipine pharmacokinetics, so patients with renal failure may take the usual starting dose [1]. Elderly patients and patients with hepatic insufficiency have decreased clearance and about 40% to 60% higher AUC, so a lower starting dose may be needed. The label states that the plasma half-life is 56 hours in impaired hepatic function and says to titrate slowly in severe hepatic impairment [1]. For children 6 to 17 years with hypertension, the label gives an effective dose of 2.5 mg to 5 mg once daily; doses above 5 mg daily have not been studied in this age group [1].

The label also warns that worsening angina and acute myocardial infarction can develop after starting or increasing the dose, particularly in severe obstructive coronary artery disease [1].