Dizziness, lightheadedness, weakness, nausea, flushing, constipation, and headache may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. To reduce the risk of dizziness and lightheadedness, get up slowly when rising from a sitting or lying position. Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor right away if you have any serious side effects, including: nausea/vomiting that doesn't stop, fainting, new or worsening symptoms of heart failure (such as shortness of breath, swelling ankles/feet, unusual tiredness, unusual/sudden weight gain), slow/irregular/pounding/fast heartbeat, mental/mood changes (such as depression, agitation), unusual dreams, severe stomach/abdominal pain, dark urine, yellowing eyes/skin. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Some products that may interact with this drug include: digoxin, fingolimod, labetalol. Other medications can affect the removal of diltiazem from your body, which may affect how this medication works. Examples include azole antifungals (such as ketoconazole), cimetidine, macrolide antibiotics (such as erythromycin), rifamycins (such as rifabutin, rifampin), St. John's wort, among others. This medication can slow down the removal of other medications from your body, which may affect how they work. Examples of affected drugs include aprepitant/fosaprepitant, asunaprevir, buspirone, colchicine, elacestrant, flibanserin, ivabradine, lomitapide, certain benzodiazepines (triazolam, midazolam), among others. Some products have ingredients that could raise your heart rate or blood pressure. Tell your pharmacist what products you are using, and ask how to use them safely (especially cough-and-cold products, diet aids, or NSAIDs such as ibuprofen/naproxen).