Fosamprenavir (Fosamprenavir) for HIV

Fosamprenavir (fosamprenavir) is a prescription medication approved by the U.S. Food and Drug Administration (FDA) to treat human immunodeficiency virus type 1 (HIV-1) infection in adults and children ages 4 weeks and older when used with other antiretroviral medicines.

How Fosamprenavir Works and How It’s Taken

Fosamprenavir is a type of HIV medicine called a protease inhibitor. In the body, Fosamprenavir is changed into amprenavir. Amprenavir blocks HIV protease, an enzyme the virus needs as part of its life cycle.

Doctors prescribe Fosamprenavir when treating HIV-1 infection in adults and children ages 4 weeks and older, together with other antiretroviral medicines.

Fosamprenavir is available as tablets and an oral suspension. Fosamprenavir tablets may be taken with or without food. Adults should take the oral suspension without food, while children should take the oral suspension with food.

Typical Dosing for HIV

For adults who have not used an HIV protease inhibitor before, Fosamprenavir may be taken in these ways:

Fosamprenavir without ritonavir — The dose is 1,400 milligrams twice daily.
Fosamprenavir with ritonavir 200 milligrams once daily — The dose is 1,400 milligrams once daily.
Fosamprenavir with ritonavir 100 milligrams once daily — The dose is 1,400 milligrams once daily.
Fosamprenavir with ritonavir 100 milligrams twice daily — The dose is 700 milligrams twice daily.

For adults who have used a protease inhibitor before, the recommended dose is Fosamprenavir 700 milligrams twice daily with ritonavir 100 milligrams twice daily. Once-daily dosing is not recommended for these adults.

For children ages 4 weeks to 18 years, the dose is based on body weight. The twice-daily oral suspension doses with ritonavir are:

Less than 11 kilograms — Fosamprenavir 45 milligrams per kilogram plus ritonavir 7 milligrams per kilogram
11 to less than 15 kilograms — Fosamprenavir 30 milligrams per kilogram plus ritonavir 3 milligrams per kilogram
15 to less than 20 kilograms — Fosamprenavir 23 milligrams per kilogram plus ritonavir 3 milligrams per kilogram
20 kilograms or more — Fosamprenavir 18 milligrams per kilogram plus ritonavir 3 milligrams per kilogram

Children should not receive more than Fosamprenavir 700 milligrams plus ritonavir 100 milligrams twice daily.

Children ages 2 and older who have not used a protease inhibitor before may also take Fosamprenavir 30 milligrams per kilogram twice daily without ritonavir.

Once-daily Fosamprenavir is not recommended for children. It is also not recommended for children younger than 6 months who have used a protease inhibitor before. Infants should receive Fosamprenavir only if they were born at 38 weeks of pregnancy or later and are at least 28 days old.

During pregnancy, Fosamprenavir 700 milligrams twice daily with ritonavir 100 milligrams twice daily should only be considered for people who were already taking this same regimen before pregnancy and whose HIV-1 viral load is below 50 copies per milliliter. Viral load should be monitored closely.

This information is based on prescribing information, but your healthcare provider may tailor your treatment plan. Always follow their guidance.

Fosamprenavir Side Effects

In clinical studies, the most common moderate to severe side effects occurred in about 2 percent to 13 percent of adults. These included:

Diarrhea
Rash
Nausea
Vomiting
Headache

In children, vomiting and neutropenia (low white blood cell counts) occurred more often than in adults.

Serious Side Effects and Warnings

Fosamprenavir can cause serious side effects that may require immediate medical attention. These include:

Severe skin reactions — Stevens-Johnson syndrome (a rare, life-threatening rash that can include blistering, peeling skin, or sores in the mouth) can occur.
Sulfa allergy — Fosamprenavir contains a sulfonamide and should be used with caution in people with a known sulfonamide allergy. The potential for cross-sensitivity is unknown.
Liver problems — Liver enzyme levels can increase, especially in people with hepatitis B or hepatitis C.
Diabetes or high blood sugar — New or worsening diabetes or high blood sugar can occur.
Immune reconstitution syndrome — Immune system changes after starting HIV treatment can cause new or worsening symptoms of infection and sometimes autoimmune conditions.
Increased body fat — Changes in body fat distribution have been reported with protease inhibitors.
High cholesterol or triglycerides — Blood fat levels can increase.
Bleeding problems in people with hemophilia — Unexpected bleeding episodes can occur.
Nephrolithiasis (kidney stones) — Cases have been reported with Fosamprenavir.

This information is based on the prescribing information. Always follow your healthcare provider’s guidance.

Get medical help right away if you think you are having a serious reaction.

What To Know Before Taking Fosamprenavir

Your doctor will order blood tests before you start Fosamprenavir and during treatment to check your liver function and blood fat levels.

Before starting Fosamprenavir, tell your doctor if you:

Are allergic to medicines that contain sulfa
Are allergic to Fosamprenavir or any of its ingredients
Have or have had liver problems, including hepatitis B or C virus infection
Have kidney problems
Have high blood sugar (diabetes)
Have hemophilia
Have high cholesterol

Tell your doctor about all medicines you take, including prescription drugs, over-the-counter medicines, vitamins, and herbal supplements.

Do not take Fosamprenavir with:

Alfuzosin
Rifampin
Certain ergot medicines
Cisapride
St. John’s wort
Lomitapide
Lovastatin
Simvastatin
Pimozide
Delavirdine
Sildenafil used for pulmonary arterial hypertension
Triazolam
Oral midazolam

If you take Fosamprenavir with ritonavir, you also should not take:

Flecainide
Propafenone
Lurasidone

Do not miss doses or let your supply of Fosamprenavir run out. The amount of virus in your blood may increase, and the virus may become harder to treat.

If you miss a dose, take it as soon as you remember. Do not take two doses at the same time or take more than prescribed.

Fosamprenavir may make combined hormonal birth control less effective. Use another effective form of birth control or add a barrier method during treatment.

A pregnancy exposure registry monitors pregnancy outcomes in people exposed to Fosamprenavir. Healthcare providers can register pregnancies through the Antiretroviral Pregnancy Registry or by calling 800-258-4263.

Do not breastfeed while taking Fosamprenavir. HIV-1 can pass to a baby through breast milk, and it is not known whether Fosamprenavir passes into human breast milk. Talk with your healthcare provider about the best way to feed your baby.

Community FAQs

These answers are fact-checked by our editorial staff.

How effective is Fosamprenavir for HIV?

In one 48-week study of adults starting HIV treatment, 66 percent of people taking Fosamprenavir twice daily lowered their HIV-1 level to below 400 copies per milliliter and kept it there. About 57 percent lowered it to below 50 copies per milliliter and kept it there.

In another 48-week study, 69 percent of people taking Fosamprenavir with ritonavir once daily lowered their HIV-1 level to below 400 copies per milliliter and kept it there. About 58 percent lowered it to below 50 copies per milliliter and kept it there.

Among adults who had previously taken protease inhibitors, a type of HIV medicine, 58 percent of those taking Fosamprenavir with ritonavir twice daily lowered their HIV-1 level to below 400 copies per milliliter and kept it there through 48 weeks.

How long does Fosamprenavir take to work for HIV?

Clinical studies reported HIV results at 24 weeks in children and through 48 weeks in adults. The prescribing information does not say exactly how soon Fosamprenavir starts lowering HIV levels in an individual person.

What tests or monitoring are needed with Fosamprenavir for HIV?

Your doctor should do regular blood tests before and during treatment with Fosamprenavir. These may include:

Liver tests — Fosamprenavir can raise liver enzyme levels, and people with hepatitis B or C may need closer monitoring.
Cholesterol and triglyceride tests — These should be done before treatment and from time to time during treatment.
Other blood tests — Changes in blood sugar and blood cell counts have been reported.

During pregnancy, HIV-1 viral load should be monitored closely to make sure the virus stays suppressed. Your doctor should also review your medicines for possible drug interactions before and during Fosamprenavir treatment.