Tybost (cobicistat) - HIV Drug Information

Photograph courtesy Gilead Sciences


Tybost (cobicistat) is a drug classified as a CYP3A inhibitor used in the treatment of HIV. Tybost works by "boosting" plasma concentration of the antiretroviral drugs, Vitekta (elvitegravir), Reyataz (atazanavir) and Prezista (darunavir), when used in combination antiretroviral therapy (cART).

Tybost is a component of three fixed dose combinations drugs, Stribild (elvitegravir + cobicistat + tenofovir + emtricitabine), Evotaz (atanazavir + cobicistat) and Prezcobix (darunavir + cobicistat),

Tybost was approved by the U.S. Food and Drug Administration (FDA) in September 2014 for both experienced and newly diagnosed adult patients with HIV-1 infection.


150mg film-coated round tablet, light orange in color and embossed with the letters "GSI" on one side.


One 150mg tablet taken daily with food at the same time as either Reyataz or Prezista. Therapy must include one or more other, accompanying antiretroviral drug(s).


Before starting, patients should be tested to assess their estimated creatinine clearance as Tybost can potentially impair renal (kidney) function, most often in patients with known or undiagnosed renal dysfunction. When coadministered with Viread (tenofovir)—a drug also known to cause renal impairment in some—it is recommended that patients be tested for estimated creatinine clearance, urine glucose and urine protein.

Common side effects:

In Phase III human clinical studies, a number of potential drug side effects have been identified when using Tybost.

The most common adverse events, reported in 2% or less of patients, were:

  • Jaundice (yellowing of the skin and eyes)
  • Ocular icterus (yellowing of the eyes)
  • Nausea
  • Diarrhea

Less common side effects include fatigue, headache, insomnia, vomiting and upper abdominal pain. Kidney stones were also noted in some patients when Tybost was coadministered with Reyataz and Viread, although the cases were considered mild and did not result in treatment discontinuation.

Drug Contraindications:

Tybost should not be taken with the following drugs or herbal supplements:

  • Anti-arrhythmic medications: Multaq
  • Anti-cholesterol medications: Mevacor, Zocor
  • Anti-gout medication: Uroxatral
  • Anti-hypertension medications: Revatio
  • Anti-psychotic medications: Orap
  • Cancer medications: Camptosar, Campto (when coadministered with Reyataz)
  • Ergot-containing anti-migraine derivatives: DHE 45, Ergotrate, Hydergine, Migrana
  • Rifampin-based anti-tuberculosis medications: Mycobutin, Rifater, Rifamate, Rimactane, Rifadin, Priftin
  • Sedatives: Dormicum, Halcion, Hypam, Hyponvel, Trilam, Versed
  • St. John's Wort

Antiretroviral Drug Interactions:

The use of Tybost with following antiretroviral drugs may result either in the loss of therapeutic effect of the accompanying drug or an increase of side effects:

    Other Interactions or Considerations:

    • Inform your doctor of any known or suspected kidney condition you may have prior to starting Tybost.
    • Antacids with aluminum or magnesium hydroxide can prevent Tybost/Reyataz from being properly absorbed. If you are using an over-the counter antacid (e.g. Rolaids, Tums, Milk of Magnesia), it is suggested that it be taken either two hours before or after a dose of Tybost/Reyataz.
    • Proton pump inhibitors (e.g., Losec, Prilosec, Prevacid, Nexium, Pariet) can similarly block absorption of Tybost/Reyataz. It is suggested that they be taken either 12 hours before or after a dose of Tybost/Reyataz.
    • Erectile dysfunction medication (Viagra, Levitra, Cialis) should be used with caution as Tybost can boost the level of adverse side effects, including visual disturbances, hypotension, and priapism (painful, sustained erections). It is recommended that Viagra dosage be decreased to no more than 25mg in 48 hours; Levitra dosage be decreased to no more than 2.5mg  in 72 hours; and Cialis dosage be decreased to no more than 10mg in 72 hours.
    View Article Sources
    • H2-receptor antagonists used to block stomach acid (e.g., Tagamet, Pepcid, Axid, Zantac) can also block absorption of Tybost/Reyataz. It is suggested that they be taken either with Tybost/Reyataz or, alternately, 10 hour before or after a dose of Tybost/Reyataz.
    • Sources:
    • Gilead Sciences. "Tybost (cobicistat) - FULL PRESCRIBING INFORMATION." Foster City, California; accessed February 3, 2015.