Cobicistat belongs to the class of medications called pharmacokinetic enhancers. It is used along with atazanavir, an HIV protease inhibitor, to increase the amount of atazanavir available to treat the infection caused by the human immunodeficiency virus (HIV). HIV is the virus responsible for acquired immune deficiency syndrome (AIDS). HIV infection destroys CD4 (T) cells, which are important to the immune system. The immune system helps fight infections.

Cobicistat works by slowing down the enzyme that breaks down atazanavir in the liver, which therefore causes an increased amount, or a "boost" of atazanavir in the body.

Cobicistat does not treat HIV infection directly. Using cobicistat in combination with atazanavir does not cure AIDS and does not reduce the risk of passing HIV to others through sexual contact or blood contamination. It is used in combination with other anti-HIV medications to slow further growth or reproduction of HIV. It also seems to slow down the destruction of the immune system. This may help to delay the development of problems such as infections related to AIDS or HIV disease.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

The recommended dose of cobicistat is 150 mg taken once daily. It should be taken with food and atazanavir. Swallow the tablet whole, with plenty of water.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you stop taking cobicistat, the amount of atazanavir in your body may decrease and it may not be as effective against the HIV virus.

If you miss a dose, take it as soon as possible with food and continue with your regular schedule. If it is less than 12 hours until your next dose, skip the missed dose and continue with your regular dosing schedule.

Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature in the original container. Protect it from light and moisture, and keep it out of the reach of children.

Each orange, round, biconvex, film-coated tablet, debossed with "GSI" on one side and plain-faced on the other side contains 150 mg of cobicistat. Nonmedicinal ingredients: silicon dioxide, microcrystalline cellulose, croscarmellose sodium, and magnesium stearate; coating: polyethylene glycol, polyvinyl alcohol, talc, titanium dioxide, sunset yellow FCF (FD&C Yellow No. 6) aluminum lake, and iron oxide yellow.

Do not take this medication if you:

• are allergic to cobicistat or any ingredients of the medication
• are taking any other medications that contain cobicistat (combination products to treat HIV)
• are taking any of the following medications:
• alfuzosin
• apixaban
• astemizole
• colchicine
• dronedarone
• drospirenone/ethinyl estradiol
• elbasvir
• ergot-containing medications (e.g., dihydroergotamine, ergonovine, ergotamine, methylergonovine)
• grazoprevir
• irinotecan
• lomitapide
• lovastatin
• lurasidone
• midazolam
• nevirapine
• pimozide
• ranolazine
• rifampin
• rivaroxaban
• St. John's wort
• salmeterol
• seizure medications: carbamazepine, phenobarbital, phenytoin
• sildenafil (when used to treat lung problems)
• simvastatin
• terfenadine
• triazolam

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 this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. 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.

• nausea
• rash

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:

• unexpected muscle pain
• yellowing of the skin or eyes

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

• symptoms of a serious allergic reaction such as swelling of the face or throat, hives, or difficulty breathing

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 this medication.

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 this medication.

Fat redistribution: Over time, this medication may change how fat is distributed in your body and may change your body shape. You may notice increased fat in the upper back and neck, breast, around the back, chest, and stomach area; or loss of fat from the legs, arms, and face. The long-term effects of this are not known.

Hepatitis B and hepatitis C infection: The safety and effectiveness of using cobicistat with co-existing liver disorders have not well been studied. People with chronic hepatitis B or C who are also using antiretroviral medications are at an increased risk of developing severe and possibly fatal liver problems. Your doctor may test for hepatitis B and hepatitis C before prescribing this medication. If you have a history of other liver disease, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Immune reconstitution inflammatory syndrome: This medication may cause immune reconstitution syndrome, where signs and symptoms of inflammation from previous infections appear. These symptoms occur soon after starting anti-HIV medication and can vary. They are thought to occur as a result of the immune system improving and being able to fight infections that have been present without symptoms (such as pneumonia, herpes, hepatitis, or tuberculosis). Report any new symptoms to your doctor immediately.

Kidney function: Cobicistat may cause a slight decrease in kidney function. While this is not normally a problem, if it is combined with another anti-HIV medication, tenofovir, it may cause kidney failure. If you have decreased kidney function or kidney disease, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed. This medication is not recommended for people with severely reduced kidney function.

Liver function: The safety and effectiveness of using cobicistat with liver disorders have not been well studied. People with chronic hepatitis B or C who are also using antiretroviral medications are at an increased risk of developing severe and possibly fatal liver problems. If you have decreased liver function or liver disease, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed. This medication is not recommended for people with severely reduced liver function.

Pregnancy: This medication should not be used during pregnancy unless the benefits outweigh the risks. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: It is not known if cobicistat passes into breast milk. Women who have HIV infection are cautioned against breast-feeding because of the risk of passing HIV to a baby who does not have the infection.

Children: The safety and effectiveness of using this medication have not been established for children.

There may be an interaction between cobicistat and any of the following:

• aliskiren
• alpha-blockers (e.g., alfuzosin, doxazosin, silodosin, tamsulosin)
• antiarrhythmics (e.g., amiodarone, disopyramide, dronedarone, lidocaine, mexiletine, procainamide, quinidine)
• anti-cancer medications (e.g., cabazitaxel, docetaxel, doxorubicin, etoposide, ifosfamide, irinotecan, trabectedin, vincristine)
• certain antihistamines (e.g., bilastine, chlorpheniramine, hydroxyzine, rupatadine)
• anti-psychotic medications (e.g., chlorpromazine, clozapine, haloperidol, olanzapine, quetiapine, risperidone)
• aprepitant
• apixaban
• "azole" antifungals (e.g., itraconazole, ketoconazole, voriconazole)
• benzodiazepines (e.g., chlordiazepoxide, clonazepam, diazepam, lorazepam, triazolam)
• beta-blockers (e.g., carvedilol, metoprolol, propranolol)
• birth control pills
• bosentan
• bromocriptine
• buprenorphine
• buspirone
• calcitriol
• calcium channel blockers (e.g., amlodipine, diltiazem, felodipine, nifedipine, verapamil)
• cannabis
• clindamycin
• clopidogrel
• colchicine
• conivaptan
• corticosteroids (e.g., betamethasone, budesonide, dexamethasone, fluticasone, hydrocortisone, mometasone, prednisone)
• cyclosporine
• dabigatran
• darifenacin
• digoxin
• domperidone
• dutasteride
• elagolix
• enzalutamide
• ergot alkaloids (e.g., dihydroergotamine, ergonovine,  ergotamine, methylergonovine)
• estrogens (e.g., conjugated estrogen, estradiol, ethinyl estradiol)
• eszopiclone
• everolimus
• galantamine
• guanfacine
• hepatitis C antivirals (e.g., glecaprevir and pibrentasvir, voxilaprevir)
• HIV non-nucleoside reverse transcriptase inhibitors (NNRTIs; e.g., efavirenz, etravirine, nevirapine, rilpivirine)
• HIV protease inhibitors (e.g., atazanavir, indinavir, ritonavir, saquinavir)
• ivabradine
• lemborexant
• lomitapide
• lumacaftor and ivacaftor
• macitentan
• macrolide antibiotics (e.g., clarithromycin, erythromycin)
• maraviroc
• mefloquine
• methadone
• mifepristone
• mirtazapine
• modafinil
• narcotic pain relievers (e.g., codeine, fentanyl, hydrocodone, tramadol)
• naloxegol
• orlistat
• oxybutynin
• phosphodiesterase-5 inhibitors (e.g., sildenafil, tadalafil, vardenafil)
• pimecrolimus
• prasugrel
• praziquantel
• progestins (e.g., dienogest, levonorgestrel, medroxyprogesterone, norethindrone)
• protein kinase inhibitors (e.g., bosutinib, dasatinib, imatinib, nilotinib)
• proton pump inhibitors (e.g., lansoprazole, omeprazole)
• quinine
• ranolazine
• repaglinide
• rifabutin
• rifampin
• riociguat
• rivaroxaban
• roflumilast
• romidepsin
• St. John's wort
• salmeterol
• sarilumab
• saxagliptin
• seizure medications (e.g., carbamazepine, gabapentin, levetiracetam, phenytoin, rufinamide, topiramate)
• selective serotonin agonists ("triptan" migraine medications; e.g., almotriptan, eletriptan)
• siltuximab
• sirolimus
• solifenacin
• "statin" anti-cholesterol medications (e.g., atorvastatin, lovastatin, simvastatin)
• tacrolimus
• tenofovir
• ticagrelor
• tocilizumab
• tolterodine
• tolvaptan
• trazodone
• vilanterol
• warfarin
• zolpidem
• zopiclone

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.