Patients taking nelfinavir (Viracept) should not take the following drugs:

• Amiodarone (Cordarone X), due to heart problems.
• Astemizole, due to heart problems.
• Cisapride, due to heart problems.
• Ergotamine tartrate (Cafergot / Migril), due to heart problems.
• Halofantrine.
• Hypericin (St John’s wort), due to decreased nelfinavir levels.
• Lovastatin, due to the risk of lovastatin-associated side-effects.
• Lumefantrine.
• Midazolam (Hypnovel), due to the risk of sedation or breathing problems.
• Quinidine (Kinidin Dureles), due to heart problems.
• Pimozide (Orap), due to heart problems.
• Rifampicin (Rifadin / Rimactane), due to increased levels of nelfinavir.
• Simvastatin (Zocor), due to simvastatin-associated side-effects^[1].
• Terfenadine, due to heart problems.
• Triazolam, due to the risk of sedation or breathing problems.

Indinavir (Crixivan), saquinavir (Invirase) and amprenavir (Agenerase) also increase the concentrations of nelfinavir, and vice versa. No dose adjustment is required for amprenavir or saquinavir, but indinavir (Crixivan) should be taken at 1200mg with 1250mg nelfinavir twice a day. Lopinavir levels may also be reduced in people taking ritonavir-boosted lopinavir (Kaletra) with nelfinavir. Measuring drug levels in the blood may be useful.

Efavirenz (Sustiva) can cause elevations in blood nelfinavir levels when it is given with low-dose ritonavir (Norvir)^[2]. Without ritonavir, efavirenz does not affect nelfinavir levels, but there is some evidence that nelfinavir concentrations may begin to fall over time in people who are taking nelfinavir and efavirenz^[3]. Monitoring drug concentrations to ensure adequate drug exposure may be useful.

A number of other drugs interact with nelfinavir and may require dose adjustements:

• Atorvastatin (Lipitor) should be used with caution in patients taking nelfinavir^[4].
• Carbamazepine (Tegretol) may reduce nelfinavir concentrations and nelfinavir can cause increases in carbamazepine levels, leading to drowsiness and unsteadiness. A dose reduction of 25 to 50% of carbamazepine should be considered, and drug level monitoring used, if possible^[5].
• Methadone hydrochloride (Methadose) levels are reduced by nelfinavir, but this does not necessarily cause withdrawal symptoms to develop^[6].
• Phenobarbital may reduce nelfinavir concentrations and should be co-administered with caution.
• Phenytoin (Epanutin) levels may be reduced by nelfinavir. Dose increases may be necessary.
• Pravastatin sodium (Lipostat) levels are reduced by nelfinavir. A dose increase may be necessary^[7].
• Rifabutin (Mycobutin) increases nelfinavir levels, and nelfinavir can decrease rifabutin levels. A reduction of the dose of rifabutin from 300 to 150mg once daily is recommended.
• Sildenafil (Viagra) levels are increased in patients taking nelfinavir. Sildenafil should be started at a reduced dose of 25mg.
• 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 the oral contraceptives norethindrone and ethinylestradiol are reduced by nelfinavir, leading to an increased likelihood of pregnancy^[8]. Alternative methods of contraception are advised.

There is also evidence that levels of the chemotherapy drug combination of adriamycin, cyclophosphamide (Endoxana), vincristine sulphate (Oncovin) and prednisolone (CHOP), used to treat non-Hodgkin’s lymphoma, may be lower than expected in patients taking nelfinavir^[9].