Do NOT use Isoptin if:

• you are allergic to any ingredient in Isoptin
• you have certain heart problems (eg, left ventricular dysfunction, sick sinus syndrome, second- or third-degree heart block and do not have a pacemaker), very low blood pressure, or moderate to severe congestive heart failure (CHF)
• you have atrial fibrillation or flutter and a pre-excitation syndrome (extra conduction pathway in the heart), such as Wolff-Parkinson-White (WPW) syndrome or Lown-Ganong-Levine (LGL) syndrome
• you have shock due to serious heart problems
• you are taking dofetilide, or you have taken disopyramide within the past 48 hours.

Contact your doctor or health care provider right away if any of these apply to you.

Some medical conditions may interact with Isoptin. Tell your doctor or pharmacist if you have any medical conditions, especially if any of the following apply to you:

• if you are pregnant, planning to become pregnant, or are breast-feeding
• if you are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
• if you have allergies to medicines, foods, or other substances
• if you have heart failure or a recent heart attack with lung congestion, low blood pressure, or heart problems (eg, a very slow heart rate, heart block, heart valve disease)
• if you have kidney or liver problems, muscular dystrophy, or a neuromuscular disease
• if you are taking another blood pressure medicine or you are being treated for cancer.

Some medicines may interact with Isoptin. Tell your health care provider if you are taking any other medicines, especially any of the following:

• Beta-blockers (eg, propranolol), clonidine, disopyramide, ketolides (eg, telithromycin), macrolides (eg, erythromycin), or ritonavir because they may increase the risk of Isoptin's side effects
• Phenobarbital, rifampin, or St. John's wort because they may decrease Isoptin's effectiveness
• Aldosterone blockers (eg, eplerenone), aspirin, carbamazepine, colchicine, cyclosporine, digoxin, dofetilide, doxorubicin, , dronedarone, eletriptan, erythromycin, everolimus, flecainide, HMG-CoA reductase inhibitors (eg, lovastatin, simvastatin), lithium, narcotic pain relievers (eg, fentanyl), paclitaxel, quinazolines (eg, terazosin), quinidine, ranolazine, theophyllines, or tolvaptan because the risk of their side effects may be increased by Isoptin.

This may not be a complete list of all interactions that may occur. Ask your health care provider if Isoptin may interact with other medicines that you take. Check with your health care provider before you start, stop, or change the dose of any medicine.

Important safety information:

• Isoptin may cause drowsiness or dizziness. These effects may be worse if you take it with alcohol or certain medicines. Use Isoptin with caution. Do not drive or perform other possible unsafe tasks until you know how you react to it.
• Do not drink alcohol or use medicines that may cause drowsiness (eg, sleep aids, muscle relaxers) while you are using Isoptin; it may add to their effects. Ask your pharmacist if you have questions about which medicines may cause drowsiness.
• Isoptin may cause dizziness, lightheadedness, or fainting; alcohol, hot weather, exercise, or fever may increase these effects. To prevent them, sit up or stand slowly, especially in the morning. Sit or lie down at the first sign of any of these effects.
• Patients who take medicine for high blood pressure often feel tired or run down for a few weeks after starting treatment. Be sure to take your medicine even if you may not feel "normal." Tell your doctor if you develop any new symptoms.
• Tell your doctor or dentist that you take Isoptin before you receive any medical or dental care, emergency care, or surgery.
• Lab tests, including liver function, kidney function, complete blood cell counts, and blood pressure, may be performed while you use Isoptin. These tests may be used to monitor your condition or check for side effects. Be sure to keep all doctor and lab appointments.
• Use Isoptin with caution in the elderly; they may be more sensitive to its effects.
• Isoptin should not be used in children younger 18 years; safety and effectiveness in these children have not been confirmed
• Pregnancy and breast-feeding: If you become pregnant, contact your doctor. You will need to discuss the benefits and risks of using Isoptin while you are pregnant. Isoptin are found in breast milk. If you are or will be breast-feeding while you use Isoptin, check with your doctor. Discuss any possible risks to your baby.

All medicines may cause side effects, but many people have no, or minor, side effects.

Check with your doctor if any of these most common side effects persist or become bothersome:

Constipation; dizziness; fatigue; headache; lightheadedness; nausea.

Seek medical attention right away if any of these severe side effects occur:

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); chest pain; fainting; lightheadedness, especially when standing; severe dizziness; shortness of breath; swelling of the feet or hands; symptoms of liver problems (eg, yellowing of the skin or eyes, dark urine, pale stools, severe or persistent stomach pain, fever, general feeling of being unwell); unusually fast, slow, or irregular heartbeat.

This is not a complete list of all side effects that may occur. If you have questions about side effects, contact your health care provider.