Justin Moore is an infectious disease clinical pharmacist at Northwestern Memorial Hospital in Chicago. The information here represents his knowledge and clinical experience. Moore was not involved in the development of ensitrelvir (Xocova).
Until now, most COVID-19 medicines were designed for use after you got sick. Ensitrelvir (Xocova) is the first oral antiviral approved to help prevent COVID after you’ve been exposed to someone with the virus. That’s called post-exposure prophylaxis, or PEP.
Justin Moore, PharmD, practices antimicrobial stewardship at Northwestern Medicine. He works as part of the medical team to help choose the right antivirals or antibacterials to help sick people get better.
We talked with Moore about who ensitrelvir may help, the way it works, and how to use it alongside vaccination and other prevention tools to protect against COVID.
Who Is This Treatment For?
Ensitrelvir is approved for adults and young people ages 12 and older after contact with someone who has COVID. For best results, you’re supposed to use it as soon as possible after exposure, but at least within three days of being around someone with a known infection.
In everyday life, the people who may benefit most are those at higher risk for worse outcomes if they get infected. That includes adults 65 and older, people who are immunocompromised, and people with health conditions that make COVID or long COVID more of a concern.
There may also be times when someone who’s not high-risk might want to take extra steps not to get sick after exposure, such as before a major trip, a wedding, or while caring for someone vulnerable.
That said, the main study looked at a very common situation where one person in a household had COVID, and other people in the home didn’t have symptoms when they started the medication.
If your partner, child, roommate, or another household member tests positive and you want to lessen your chances of developing symptoms, that’s the kind of setting the researchers studied the drug in.
What’s Different About This Drug?
Ensitrelvir is an antiviral. That means it works against the virus itself.
More specifically, it blocks a viral enzyme that SARS-CoV-2 needs to make more copies of itself. If the virus that causes COVID can’t replicate, it may not build up enough in your body to cause symptoms.
That’s what makes this drug different from the COVID medicines many people already know. Nirmatrelvir-ritonavir (Paxlovid) and molnupiravir (Lagevrio) are used to treat COVID after symptoms start. Ensitrelvir is the first antiviral tablet approved in the U.S. to help prevent symptomatic COVID after a known exposure.
Other medicines have been studied for this kind of prevention. But they didn’t show the same level of benefit.
In the past, monoclonal antibodies were also used for prevention in some people. Those treatments became less useful as the virus changed. They also had to be given as an injection or through a vein, which made them harder to use.
A pill is easier to take quickly after exposure, and antivirals work best when taken early.
Does Ensitrelvir Replace the Need for a COVID Vaccine?
No. I can’t stress this enough: Antivirals are another tool, not a replacement for vaccines.
Vaccines and antivirals work in different ways.
A vaccine helps your body build defenses before you’re exposed, so your immune system is better prepared if the virus gets in your body later. Vaccines are still the best way to lower your chances of getting very sick, going to the hospital, or dying from COVID.
An antiviral works after exposure. Once the virus gets into your body, the virus tries to use your cells to make more copies of itself. Ensitrelvir is designed to block that copying process early.
So a COVID vaccine lowers the risk of a bad outcome. Ensitrelvir may lower the chance that a known exposure turns into a symptomatic infection.
You can and should still use other prevention steps, including keeping your distance from sick people, washing your hands, and masking when it makes sense.
How Effective Is This Treatment?
In the SCORPIO-PEP trial, people who took ensitrelvir were 67% less likely to get symptomatic COVID after household exposure compared to people who didn’t get the drug.
About 3% of people who took ensitrelvir got symptomatic COVID, compared with about 9% of those who got a placebo.
Those numbers are useful in two ways. First, the results tell us the drug worked well in the study. Second, the actual numbers show that most people in both groups didn’t develop symptomatic COVID, but fewer people got sick when they took ensitrelvir.
When you take the drug is also key. In the study, people started treatment within 72 hours of when the infected household member’s symptoms began. The benefit appears stronger when people start even earlier, especially within the first 48 hours.
Knowing exactly when to start the medication can be tricky in real life. Someone in your home may have mild symptoms for a day or two before they test positive. By then, your exposure clock may already be ticking.
So if you think ensitrelvir may be something you’d want after a household exposure, it helps to talk with your doctor or pharmacist before you need it.
How Do You Take Ensitrelvir?
You take it by mouth for five days, with or without food.
The first day is a loading dose, which means you take more medicine up front. You take three tablets on day 1, then one tablet each day on days 2 through 5.
This is meant to be a short course after exposure. It’s not something you take every day for long-term prevention. And in the U.S., it’s approved to help prevent COVID after exposure, not to treat active COVID symptoms.
Are There Any Side Effects?
In the main prevention study, side effects happened at similar rates in people who took ensitrelvir and people who took the placebo.
The most common side effects reported with ensitrelvir were things like headache, diarrhea, and cough. Most side effects were mild and overlapped with the kind of symptoms you might get during a viral infection.
One thing people often worry about with COVID antivirals is taste change. With Paxlovid, some people noticed a strong metallic or bitter taste. In the ensitrelvir prevention study, that wasn’t a big problem tied to the drug. That should make it easier to finish the full five-day course.
Is This Treatment Safe?
Based on the study data, ensitrelvir was generally well-tolerated. That means side effects weren’t so bad that people had to stop treatment. There were no major safety concerns.
But “safe” always depends on the person taking it.
The biggest safety issue to know about is drug interactions. Ensitrelvir can affect how your body processes some medications, and some medication can affect how ensitrelvir works.
That’s why your doctor or pharmacist needs to review everything you take before you start it. That includes prescriptions, over-the-counter medicines, vitamins, and herbal supplements.
Sometimes an interaction can be managed by pausing another medicine or changing the dose for a few days. But some medicines shouldn’t be taken with ensitrelvir at all. Don’t try to sort that out on your own.
This drug also may not be right during pregnancy or while breastfeeding. If you’re pregnant, planning to become pregnant, or breastfeeding, tell your health care provider before taking it.
Do You Need to Follow Up With Your Doctor?
Not always.
If you take ensitrelvir, never develop symptoms, and feel fine, you may not need a follow-up visit just because you took the medication.
But you should check in with a health care provider if you start to feel worse, get symptoms that worry you, or get better and then worse again.
In some rare cases, you can have more than one infection at the same time. You can also get a bacterial infection on top of a viral one. Ensitrelvir won’t treat that. And similarly, antibiotics don’t treat viral infections.
If your illness doesn’t follow a steady, improving course, I’d suggest you check in with your doctor.
Will Insurance Cover It?
We don’t know yet how every insurance plan will handle ensitrelvir.
Historically, many COVID antivirals have been covered by insurance. I’d expect many plans to cover ensitrelvir, too, but access may vary at first.
Coverage often takes time to sort out after a new drug is approved. Medicare, Medicaid, and private insurance plans may make different decisions, and some plans may have rules about when they’ll cover it.
Cost and pharmacy availability will play a big part in how easy it is to get in the real world. We’ll just have to wait and see how that goes.
How Can I Find Out If Ensitrelvir Is Right for Me?
Start with your doctor or pharmacist.
Because ensitrelvir works best when started quickly (within the first three days), you don’t want to wait until someone in your home tests positive and then spend a week trying to figure out how to get it.
Your pharmacist can be a practical first step. They’re medication experts and can help you check for drug interactions. You may be able to call or message your pharmacy and ask whether ensitrelvir might be safe with your medication list.
Your primary care doctor or regular clinic is also important, since they may be the one to prescribe it. You might ask ahead of time: “If someone in my household gets COVID, would ensitrelvir be an option for me? And how would I get it quickly?”
How Carefully Was Ensitrelvir Studied?
The main study was a randomized, double-blind, placebo-controlled trial. That’s one of the strongest ways to test a medicine.
Randomized means people were assigned by chance to get either ensitrelvir or a placebo. Double-blind means neither the participants nor the researchers knew who got which one during the study. Placebo-controlled means the drug was compared with an inactive treatment.
The study included more than 2,000 household contacts ages 12 and older who had been exposed to someone with symptomatic COVID. The people taking part didn’t have symptoms and tested negative at the start.
Researchers then tracked who went on to get symptomatic COVID.
Ensitrelvir has also been studied in other COVID settings, including as a treatment for mild to moderate COVID in Japan. So while this prevention use is new to the U.S., the drug type and research aren’t brand new.
Still, studies don’t tell us everything. We’ll have to wait and see how easy this drug is to access and which groups benefit most.
For now, the big takeaway is that ensitrelvir gives people a new way to act early after a known COVID exposure. It may not be right for everyone. But for people at higher risk or those trying hard to avoid infection after someone close to them gets sick, it may fill an important gap.
