Pharmacokinetic enhancer (CYP3A4 inhibitor) Pregnancy: eMC §4.6: there are no or limited clinical data with cobicistat in pregnant women; animal studies do not indicate reproductive toxicity. Treatment with cobicistat plus atazanavir or darunavir during pregnancy results in lower atazanavir or darunavir exposure, which may be associated with an increased risk of virological failure and of mother-to-child transmission of HIV. Therapy with cobicistat and atazanavir or darunavir should NOT be initiated during pregnancy, and women who become pregnant on such therapy should be switched to an alternative regimen. Breast-feeding: it is unknown whether cobicistat/metabolites are excreted in human milk (they are excreted in animal milk) and a risk to the infant cannot be excluded, so the product should not be used during breast-feeding; it is recommended that women living with HIV do not breast-feed.

Cobicistat

Brand names: Tybost

Cobicistat is a pharmacokinetic enhancer (booster) used in HIV therapy to increase exposure to certain antiretrovirals such as atazanavir, darunavir and elvitegravir; it has no antiviral activity of its own.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 150 mg once daily, given in combination with atazanavir 300 mg once daily or darunavir 800 mg once daily

Route: Oral - taken with food; the film-coated tablet should not be chewed or crushed

Frequency: Once daily

eMC §4.2 (Tybost 150 mg film-coated tablet). Therapy should be initiated by a physician experienced in the management of HIV infection. Cobicistat is used in combination with atazanavir or darunavir, therefore the atazanavir or darunavir SPC should also be consulted - the SPC gives only these two boosted regimens (Table 1, adults: cobicistat 150 mg once daily with either atazanavir 300 mg once daily or darunavir 800 mg once daily). MISSED DOSE: if a dose is missed within 12 hours of the time it is usually taken, the patient should take it with food as soon as possible and resume the normal dosing schedule in combination with atazanavir or darunavir; if missed by more than 12 hours, the missed dose should not be taken and the usual schedule simply resumed. ELDERLY: no data are available on which to make a dose recommendation for patients over 65 years. HEPATIC IMPAIRMENT: no dose adjustment in mild (Child-Pugh A) or moderate (Child-Pugh B) impairment; not studied in severe impairment (Child-Pugh C) and use is not recommended in those patients. ADOLESCENTS (SPC Table 2, aged 12 years and older weighing at least 35 kg): cobicistat 150 mg once daily with atazanavir 300 mg once daily for weight 35 kg and above, or with darunavir 800 mg once daily for weight 40 kg and above - this is a fixed dose, not a per-kg dose, which is why no structured paediatric dose is given. Safety and efficacy of cobicistat with atazanavir in children aged 0 to less than 12 years or weighing less than 35 kg, and with darunavir in children aged 0 to less than 12 years or weighing less than 40 kg, have not been established and no data are available. Verify any under-18 use against a children's formulary.

Dose adjustments

Renal

eMC §4.2: no dose adjustment of cobicistat is required for patients with renal impairment, including severe renal impairment. Cobicistat has not been studied in patients receiving dialysis, so no recommendation can be made for them. Cobicistat decreases estimated creatinine clearance through inhibition of tubular secretion of creatinine, and should NOT be initiated in patients with creatinine clearance less than 70 ml/min if any co-administered agent (e.g. emtricitabine, lamivudine, tenofovir disoproxil or adefovir) requires dose adjustment based on creatinine clearance.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients (eMC §4.3)
Co-administration with medicinal products highly dependent on CYP3A for clearance and for which elevated plasma concentrations are associated with serious and/or life-threatening events, including but not limited to: alfuzosin; amiodarone, quinidine; the ergot derivatives dihydroergotamine, ergometrine and ergotamine; lovastatin and simvastatin; pimozide and lurasidone; sildenafil when used for pulmonary arterial hypertension; orally administered midazolam and triazolam (eMC §4.3)
Co-administration with strong CYP3A inducers because of potential loss of therapeutic effect, including but not limited to carbamazepine, phenobarbital, phenytoin, rifampicin and St John's wort (Hypericum perforatum) (eMC §4.3)
Co-administration with dabigatran etexilate, a P-glycoprotein substrate (eMC §4.3)

Side effects

Jaundice and ocular icterus (very common) and hyperbilirubinaemia (common) - the most frequently reported reactions to cobicistat-boosted atazanavir were associated with elevated bilirubin
Nausea (very common); vomiting, diarrhoea, dyspepsia, abdominal pain, abdominal distension, flatulence and dry mouth (common)
Headache, dizziness, somnolence and dysgeusia (common); insomnia and abnormal dreams (common); depression and sleep disorder (uncommon)
Rash (common) and pruritus (uncommon); fatigue (common); pyrexia and asthenia (uncommon); myalgia (uncommon)
Hyperglycaemia and increased appetite (common); nephrolithiasis, haematuria and proteinuria (uncommon). Note (§4.8): cobicistat decreases estimated creatinine clearance by inhibiting tubular secretion of creatinine - the rise in serum creatinine due to this effect alone generally does not exceed 0.4 mg/dl

Interactions

Cobicistat is a strong mechanism-based CYP3A inhibitor and a CYP3A substrate - plasma concentrations of CYP3A-metabolised medicines (including atazanavir and darunavir) are increased, which for some products can cause serious and/or life-threatening events (eMC §4.4, see also the contraindications above)
Medicines with active metabolites formed by CYP3A may have reduced active-metabolite concentrations, potentially causing loss of therapeutic effect; CYP3A inhibitors may decrease cobicistat clearance and raise cobicistat levels (eMC §4.4)
Cobicistat is a weak CYP2D6 inhibitor and is metabolised to a minor extent by CYP2D6 - co-administration can increase plasma concentrations of CYP2D6 substrates (eMC §4.4)
Cobicistat inhibits the transporters P-gp, BCRP, MATE1, OATP1B1 and OATP1B3 - substrates of these transporters may have increased plasma concentrations (eMC §4.4)
Hormonal contraception (eMC §4.4): ethinyloestradiol concentrations are decreased with drospirenone/ethinyloestradiol plus darunavir/cobicistat, so alternative or additional contraceptive measures are recommended when oestrogen-based contraceptives are co-administered with darunavir/cobicistat; drospirenone concentrations are increased with atazanavir/cobicistat or darunavir/cobicistat, so clinical monitoring is recommended
Switching pharmacoenhancer from ritonavir to cobicistat requires caution during the first two weeks, particularly if doses of concomitant medicines were titrated or adjusted while on ritonavir - unlike ritonavir, cobicistat is not an inducer of CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19 or UGT1A1 (eMC §4.4)
The full eMC §4.5 interaction table was not captured in this bundle - clinician to review it in the SPC

Clinical monograph

How it works

It is a potent inhibitor of cytochrome P450 3A, slowing metabolism of co-administered boosted drugs and thereby raising and sustaining their plasma concentrations.

Prescribing in practice

Its strong CYP3A inhibition causes extensive, potentially serious drug interactions, so all co-medication (including statins, inhaled and intranasal corticosteroids, sedatives and ergot derivatives) must be screened before and during use.
It inhibits tubular creatinine secretion, producing a non-progressive rise in serum creatinine without a true fall in glomerular filtration.
It must be given with the specific agent it boosts and is not interchangeable with ritonavir on a like-for-like basis.

Monitoring

Monitor renal function (interpreting creatinine changes in context) and review for drug interactions at every prescribing point.

Counselling the patient

Always tell pharmacists and other prescribers you take a boosting agent before starting any new medicine, including over-the-counter products.
Take it together with food and with the antiretroviral it accompanies.
Do not stop or change doses without specialist advice.

Evidence & guidelines

Use reflects established HIV treatment guidelines and product information for boosted antiretroviral regimens.

Reference: BHIVA guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways