Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes Opsumit (Macitentan). If you are concerned about side effects, discuss the risks and benefits of Opsumit (Macitentan) with your doctor.

The following side effects have been reported by at least 1% of people taking Opsumit (Macitentan). Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• flu-like symptoms (sudden lack of energy, Fever, cough, sore throat)
• headache
• persistent cough
• stuffy nose

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• irritation of the airways (Bronchitis)
• signs of low red blood cells or Anemia (e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath)
• signs of liver problems (e.g., nausea, vomiting, Diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)
• symptoms of a urinary tract infection (e.g., painful urination, frequency, urgency, pressure in the lower pelvis, fever)

Stop taking the medication and seek immediate medical attention if any of the following occur:

• signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking Opsumit (Macitentan).

Each round, bi-convex, film-coated tablet, debossed with "10" on one side contains 10 mg Opsumit (Macitentan). Nonmedicinal ingredients: lactose monohydrate, magnesium stearate, microcrystalline cellulose, polysorbate 80, povidone, and sodium starch glycolate Type A. Film coating: polyvinyl alcohol, soya lecithin, talc, titanium dioxide and xanthan gum.

Do not take Opsumit (Macitentan) if you:

• are allergic to Opsumit (Macitentan) or any ingredients of the medication
• are pregnant or may become pregnant
• are breast-feeding

There may be an interaction between Opsumit (Macitentan) and any of the following:

• Aprepitant
• "azole" antifungals (e.g., itraconazole, ketoconazole, Voriconazole)
• boceprevir
• Bosentan
• Carbamazepine
• calcium channel blockers (e.g., Amlodipine, Diltiazem, Nifedipine, Verapamil)
• conivaptan
• Cyclosporine
• Deferasirox
• Dronedarone
• Enzalutamide
• grapefruit juice
• HIV non-nucleoside reverse transcriptase inhibitors (NNRTIs; e.g., delavirdine, efavirenz, etravirine, nevirapine)
• HIV protease inhibitors (e.g., atazanavir, indinavir, ritonavir, saquinavir)
• macrolide antibiotics (e.g., clarithromycin, erythromycin)
• Metronidazole
• mifepristone
• Mitotane
• Modafinil
• nefazodone
• Phenobarbital
• Phenytoin
• primidone
• certain protein kinase inhibitors (e.g., Crizotinib, Dabrafenib, Dasatinib, imatinib)
• rifabutin
• rifampin
• St. John's wort
• siltuximab
• simeprevir
• Stiripentol
• telaprevir
• Tocilizumab

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the Nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or Allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use Opsumit (Macitentan).

Anemia: Opsumit (Macitentan) may cause low levels of red blood cells. If you experience symptoms of reduced red blood cell count (anemia) such as shortness of breath, feeling unusually tired or pale skin, contact your doctor as soon as possible.

Kidney function: Kidney Disease or reduced kidney function may cause Opsumit (Macitentan) to build up in the body, causing side effects. If you have reduced kidney function or kidney disease, discuss with your doctor how Opsumit (Macitentan) may affect your medical condition, how your medical condition may affect the dosing and effectiveness of Opsumit (Macitentan), and whether any special monitoring is needed.

Liver function: Opsumit (Macitentan) may reduce liver function. If you have liver problems, discuss with your doctor how Opsumit (Macitentan) may affect your medical condition, how your medical condition may affect the dosing and effectiveness of Opsumit (Macitentan), and whether any special monitoring is needed. Your doctor may want to test your liver function regularly with blood tests while you are taking Opsumit (Macitentan). People with severely reduced liver function should not use Opsumit (Macitentan).

Pregnancy: Opsumit (Macitentan) may cause harm or death to the developing baby and mother if it is taken during pregnancy. Women must not become pregnant while taking Opsumit (Macitentan) or for at least one month after stopping the medication. Your doctor may routinely perform pregnancy tests while you are taking Opsumit (Macitentan) to confirm the safety of continuing to take Opsumit (Macitentan).

Women who may become pregnant must not take Opsumit (Macitentan) until pregnancy is ruled out. If you become pregnant while taking Opsumit (Macitentan), contact your doctor immediately.

Breast-feeding: It is not known if Opsumit (Macitentan) passes into breast milk. If you are a breast-feeding mother and are taking Opsumit (Macitentan), it may affect your baby. Women should not breast-feed while taking Opsumit (Macitentan).

Children: The safety and effectiveness of using Opsumit (Macitentan) have not been established for children less than 18 years of age.