A number of drugs should be avoided by patients taking indinavir (Crixivan), due to dangerous interactions, including:

• Alprazolam (Xanax).
• Amiodarone hydrochloride (Cordarone X), due to an increased risk of irregular heartbeat.
• Astemizole, due to an increased risk of irregular heartbeat.
• Atazanavir (Reyataz).
• Cisapride, due to an increased risk of irregular heartbeat.
• Ergotamine tartrate (Cafergot / Migril).
• Flecainide (Tambocor).
• Halofantrine.
• Hypericin (St John’s wort), due to reduced indinavir levels^[1].
• Lovastatin.
• Lumefantrine.
• Midazolam (Hypnovel).
• Pimozide (Orap), due to an increased risk of irregular heartbeat.
• Quinidine sulphate (Kinidin Dureles), due to an increased risk of irregular heartbeat.
• Rifampicin (Rifadin / Rimactane)^[2].
• Simvastatin (Zocor).
• Terfenadine, due to an increased risk of irregular heartbeat.
• Triazolam.
• Vardenafil (Levitra).

Indinavir interacts with some other anti-HIV drugs and dose adjustments are required:

• People taking ddI (didanosine, Videx) tablets should take them on an empty stomach one hour before or one hour after taking indinavir^[3]. However, people taking enteric-coated ddI capsules (VidexEC) can take them at the same time as indinavir.
• When combined with efavirenz (Sustiva), levels of indinavir fall. A dose increase to 1000mg indinavir every eight hours is advised, although this may increase side-effects. Concentrations of ritonavir (Norvir) are also lowered by efavirenz, so when ritonavir-boosted indinavir is taken with efavirenz, a dose adjustment to 800mg indinavir and 200mg ritonavir twice daily may be required. Monitoring the level of indinavir in the blood may be useful^[4].
• When combined with nevirapine (Viramune), levels of indinavir fall, requiring an increase in dosing to 1000mg every eight hours.
• Saquinavir (Invirase) and indinavir should be taken together cautiously.

Many other drugs and complementary therapies interact with indinavir, and may require dose adjustments, or use with caution. These include:

• Amlodipine (Istin) levels may be increased by indinavir. It should be started at a low dose^[5].
• Atorvastatin (Lipitor) levels may be increased by indinavir. It should be used with caution.
• Diltiazem hydrochloride (Tildiem / Adizem-SR / Adizem-XL / Angitil SR / Angitil XL / Calcicard CR / Dilcardia SR / Dilzem SR / Dilzem XL / Slozem / Tildiem LA / Tildiem Retard / Viazem XL / Zemtard) levels may be increased by indinavir. It should be started at a low dose^[6].
• Itraconazole (Sporanox) increases indinavir concentrations: a dose of 600mg indinavie three times daily is recommended.
• Ketoconzaole (Nizoral) increases indinavir concentrations: a dose of 600mg indinavie three times daily is recommended.
• Rifabutin (Mycobutin) concentrations are increased by indinavir, and indinavir concentrations are reduced by rifabutin. It is recommended that the dose of rifabutin is reduced by half and the dose of unboosted indinavir is increased to 1000 or 1200mg three times a day.
• Sildenafil (Viagra) levels are increased by indinavir, increasing the risk of side-effects. Patients taking indinavir should start with a reduced dose of 12.5mg sildenafil^[7].
• Tadalafil (Cialis) levels are increased by indinavir, increasing the risk of side-effects. Patients taking indinavir should half the recommended dose of tadalafil.
• Venlafaxine (Efexor / Efexor XL), an antidepressant that reduces indinavir levels^[8].

Chemotherapy for non-Hodgkin’s lymphoma consisting of adriamycin, cyclophosphamide (Endoxana), vincristine sulphate (Oncovin) and prednisolone (CHOP) can cause a reduction in indinavir levels. Testing of drug levels in the blood is recommended^[9].

In contrast to most protease inhibitors, indinavir increases the levels of the oral contraceptives norethindrone by 26% and ethinylestradiol by 24%. This can enhance their contraceptive activity^[10].

Garlic supplements have been shown to reduce blood levels of saquinavir by half, but it is not known if they have the same effect on indinavir levels. Caution should be exercised, especially if indinavir is not being boosted with ritonavir. Taking 1g or more of vitamin C a day has also been shown to reduce indinavir concentrations by 20% in healthy volunteers^[11].