Patients taking efavirenz (Sustiva) should not take the following drugs:

• Astemizole, due to the risk of irregular heartbeat.
• Bepidril.
• Cisapride, due to the risk of irregular heartbeat.
• Clarithromycin (Clarosip / Klaricid / Klaricid XL).
• Hypericin (St John’s wort), which can reduce efavirenz levels.
• Midazolam (Hyponovel), due to the risk of irregular heartbeat.
• Nevirapine (Viramune), due to an elevated risk of side-effects^[1].
• Pimozide.
• Terfenadine, due to the risk of irregular heartbeat.
• Triazolam, due to the risk of irregular heartbeat.
• Voriconazole (Vfend), due to the risk of low voriconazole levels.

Efavirenz alters the levels of most protease inhibitors. Patients taking efavirenz and protease inhibitors should usually take the protease inhibitor at non-standard doses:

• Four, rather than three capsules of ritonavir-boosted lopinavir (Kaletra) should be taken twice daily by individuals also taking efavirenz^[2].
• Indinavir (Crixivan) should be taken at the higher dose of 1000mg three times a day^[3]. If low-dose ritonavir is used to boost indinavir levels, 200mg may be required three times a day, although level of indinavir should be measured in the blood.
• Full-dose ritonavir (Norvir) should be taken at the lower dose of 500mg twice a day. The efavirenz dose should also be increased^[4].

No dose adjustment of nelfinavir (Viracept) is required when co-administered with efavirenz. However, while studies of efavirenz taken in combination with ritonavir-boosted atazanavir (Reyataz) in HIV-negative subjects have found no significant drug interaction, this may not be the case in patients with HIV. In HIV-positive patients, atazanavir clearance may be increased by efavirenz, but further studies are needed to determine whether any dose adjustments are necessary^[5][6].

The following drugs also need to be administered at non-standard doses in patients taking efavirenz:

• Atorvastatin (Lipitor) may need to be given at a higher dose^[7].
• Methadone hydrochloride (Methadose) may need to be given at a higher dose in patients taking efavirenz^[8]. The dose should be increased in 10mg steps if withdrawal symptoms appear.
• Phenytoin (Epanutin) may need to be given at lower doses to prevent reductions in efavirenz levels^[9].
• Rifabutin (Mycobutin) should be taken at a higher dose of 450 or 600mg twice a week^[10][11].
• Rifampicin (Rifadin / Rimactane) should be given at 800mg daily^[12][13][14]. However, this may not be necessary in people with low body weight^[15][16].
• Pravastatin (Lipostat) may need to be given at a higher dose^[17].
• Sildenafil (Viagra) levels are increased in patients taking nelfinavir. Sildenafil should be started at a reduced dose of 25mg.
• Simvastatin (Zocor) may need to be given at a higher dose^[18].
• Tadalafil (Cialis) should be taken at a lower dose in patients taking nelfinavir.
• Vardenafil (Levitra) should be taken at a lower dose in patients taking nelfinavir.

Levels of ciclosporin (Neoral / Sandimmun) may also be reduced following long-term administration in patients taking efavirenz^[19].

Efavirenz does not decrease levels of the hormonal contraceptive ethinylestradiol, but the use of barrier contraception is recommended in addition to oral contraceptives in order to prevent pregnancy.

Efavirenz may cause a false-positive result for a drug test for cannabis, if the CEDIA DAU Multi-Level THC assay is used. Other types of tests do not confuse efavirenz and cannabis.