Because it inhibits the P450 liver enzymes, ritonavir (Norvir) interacts with a large number of other medicines.1 Some of these drug interactions may be life-threatening.
It is extremely important for patients taking ritonavir to check with their doctor before taking any other medications whatsoever. Abbott, ritonavir’s manufacturer, has produced a card listing all the drug’s possible interactions.
Patients taking ritonavir should not take the following drugs:
- Alfuzosin
- Amiodarone (Cordarone X)
- Astemizole
- Bepidril
- Chlorazepate (Tranxene), due to a risk of sedation and breathing difficulties
- Cisapride
- Colchicine in patients with renal or hepatic impairment
- Diazepam, due to a risk of sedation and breathing difficulties
- Ergotamine tartrate (Cafergot / Migril), due to a risk of blood vessel constriction
- Estazolam, due to a risk of sedation and breathing difficulties
- Flecainide acetate (Tambocor)
- Flurazepam (Dalmane), due to a risk of sedation and breathing difficulties
- Halofantrine
- Hypericin (St John’s wort)
- Lovastatin
- Lumefantrine
- Midazolam (Hypnovel), due to a risk of sedation and breathing difficulties
- Pethidine hydrochloride (Pamergan P100)2
- Pimozide (Orap)
- Piroxicam (Feldene / Brexidol)
- Propafenone (Arythmol)
- Quinidine sulphate (Kinidin Dureles)
- Simvastatin (Zocor)
- Terfenadine
- Triazolam, due to a risk of sedation and breathing difficulties3
- Vardenafil (Levitra)
- Voriconazole (Vfend).
Co-administration of ritonavir with efavirenz (Sustiva) can increase the likelihood of side-effects such as dizziness, nausea and unusual skin sensations, as well as elevated liver enzymes.
Other drugs that should be used cautiously by people taking ritonavir include the following:
- Amitriptyline (Tryptizol, Elavil, Lentizol) levels may be increased by ritonavir.
- Atorvastatin (Lipitor) levels are increased by ritonavir.
- Atovaquone (Wellvone) levels may be reduced by ritonavir.
- Clomipramine (Anafranil) levels may be increased by ritonavir.
- Codeine phosphate levels may be reduced by ritonavir.
- Desipramine (Pertrofan) levels may be increased by ritonavir.
- Fluoxetine (Prozac) levels may be increased by ritonavir.
- Fluticasone propionate (Flixotide) levels are increased by ritonavir. This can cause symptoms of Cushing’s syndrome and suppression of the adrenal glands, including weight gain, weakening of the skin, sweating, facial hair growth and psychological disturbances.4 5
- Imipramine (Tofranil) levels may be increased by ritonavir.
- Ketoconazole (Nizoral) levels are increased by ritonavir, increasing the incidence of side-effects.
- Ketoprofen (Orudis / Oruvail) levels may be reduced by ritonavir.
- Lorazepam levels may be reduced by ritonavir.
- Maprotiline (Ludiomil) levels may be increased by ritonavir.
- Methadone hydrochloride (Methadose) concentrations are reduced by ritonavir, requiring dose escalation to avoid the risk of withdrawal symptoms developing.
- Morphine (Oramorph / Sevredol / Morcap SR / Morphegesic SR / MST Continus / MXL / Zomorph) levels may be reduced by ritonavir.
- Naproxen (Naprosyn / Synflex) levels may be reduced by ritonavir.
- Nefazodone (Dutonin) levels may be increased by ritonavir.
- Nortryptiline (Allegron) levels may be increased by ritonavir.
- Oxazepam levels may be reduced by ritonavir.
- Paroxetine (Seroxat) levels may be increased by ritonavir.
- Prednisolone levels are increased by ritonavir.6
- Propofol (Diprivan) levels may be reduced by ritonavir.
- Rifabutin (Mycobutin): levels are increased by ritonavir, so it should be taken at the lower dose of 150mg two to three times a week.7
- Sertraline (Lustral) levels may be increased by ritonavir.
- Sildenafil (Viagra) levels are increased eleven-fold by ritonavir, increasing the incidence of dangerous side-effects.8 The dose of sildenafil should be reduced to less than 25mg every 48 hours. Sildenafil is contraindicated if used for treatment of pulmonary arterial hypertension (due to dose required).
- Tadalafil (Cialis) levels are increased by ritonavir, so it should be taken at a reduced dose.
- Temazepam levels may be reduced by ritonavir.
- Trazadone (Molipaxin) levels may be increased by ritonavir.
- Venlafaxine (Effexor) levels may be increased by ritonavir.
Ritonavir decreases the blood concentrations of the oral contraceptive ethinylestradiol, so alternative forms of contraception should be used.
One case of reduced levels of valproic acid (Depakote) has also been reported in a patient taking ritonavir, resulting in re-emergence of symptoms of bipolar disorder.9 However, further data are required to confirm this link.
Ritonavir capsules and oral solution contain alcohol. They should therefore not be taken with the alcoholism treatment drug disulfiram (Antabuse) or medicines with similar modes of action, including the antibiotic metronidazole (Flagyl / Flagyl S / Metrolyl).