Diltiazem XR Side Effects: What to Expect and When to Call Your Doctor

Like all medications, Diltiazem XR (diltiazem extended-release) can cause side effects. Most patients tolerate it well, and many side effects are mild and manageable. However, a few side effects signal serious problems that require prompt medical attention. Knowing the difference could be very important for your health.

Common Side Effects of Diltiazem XR (Reported in More Than 2% of Patients)

These side effects occur frequently and are generally not dangerous on their own. However, if they are bothersome or persistent, talk to your doctor:

Lower limb edema (ankle and leg swelling) — One of the most common side effects, caused by dilation of small blood vessels and fluid shifting into tissues. Usually manageable by elevating legs; may require dose adjustment.

Headache — Often occurs when starting the medication and typically resolves within a few weeks as your body adjusts.

Dizziness — Can occur especially after standing up quickly. Rise slowly from sitting or lying positions to minimize this effect.

Fatigue — Some patients feel more tired, especially early in treatment. This often improves over time.

Sinus congestion — A stuffy or runny nose is a known side effect of calcium channel blockers.

Bradycardia (slow heart rate) — Diltiazem slows the heart rate as part of its mechanism. Mild slowing is expected. Notify your doctor if your heart rate drops below 50 beats per minute or you feel symptoms.

First-degree AV block — A minor slowing of electrical conduction through the heart, visible on ECG. Usually not clinically significant on its own.

Rash — Mild skin rash has been reported. Tell your doctor about any new rash, as some skin reactions can be serious.

Serious Side Effects: When to Call Your Doctor or Seek Emergency Care

The following side effects require immediate attention. Do not wait to see if they improve on their own:

Second- or third-degree AV block: Symptoms include severe dizziness, fainting, or a very slow or irregular heartbeat. This represents a dangerous disruption in heart electrical conduction.

Worsening heart failure: Sudden increase in shortness of breath, worsening leg swelling, or rapid weight gain (more than 2–3 lbs in 24 hours) may indicate worsening heart failure.

Liver injury: Symptoms include yellowing of the skin or eyes (jaundice), dark urine, upper right abdominal pain, or unusual fatigue. Diltiazem can cause elevated liver enzymes and, rarely, acute hepatic injury.

Severe skin reactions (Stevens-Johnson Syndrome/TEN): Blistering, peeling skin, mouth sores, or widespread rash accompanied by fever are medical emergencies. Stop the medication and seek emergency care immediately.

Severe hypotension: Feeling faint, very dizzy, or experiencing sudden weakness could indicate a dangerous drop in blood pressure. Sit or lie down and seek medical attention.

Syncope (fainting): Any episode of loss of consciousness while taking Diltiazem XR should be reported to your doctor immediately and evaluated.

Less Common Side Effects Reported After Approval

Post-market surveillance has identified additional less common side effects including: gingival hyperplasia (gum overgrowth), alopecia (hair thinning), photosensitivity, angioedema (swelling of the face or lips), and extrapyramidal symptoms (rare movement-related effects). Report any unusual new symptoms to your healthcare provider.

Side Effects to Watch for When Starting Diltiazem XR

The first few weeks on Diltiazem XR are when you're most likely to notice side effects. Your doctor may want to check your blood pressure and heart rate after starting, and may schedule an ECG to confirm your heart's electrical conduction is within normal range.

Tips for minimizing common side effects:

Take your dose at the same time each day as instructed — swallow whole, do not crush or chew

Elevate your legs at rest to reduce ankle swelling

Rise slowly from sitting or lying to prevent dizziness from position changes

Avoid grapefruit and grapefruit juice — it inhibits CYP3A4 metabolism and can increase diltiazem blood levels, worsening side effects

For information on medications and foods that can increase Diltiazem XR side effects, see Diltiazem XR Drug Interactions: What to Avoid and What to Tell Your Doctor.

If side effects are making you consider switching medications, first make sure you can easily access Diltiazem XR at a pharmacy near you using medfinder — sometimes availability issues contribute to inconsistent dosing that worsens side effects.