Antiviral medications are drugs that fight viral infections by stopping viruses from copying themselves inside your cells. Unlike antibiotics, which work against bacteria, antivirals are designed for specific viruses, so the medication you’d take for the flu is completely different from one used for HIV or hepatitis C. There are more than 90 FDA-approved antivirals available today, spanning treatments for everything from cold sores to COVID-19.
How Antivirals Work
Viruses hijack your cells to reproduce, and antivirals interrupt that process at different stages. Some block the virus from entering your cells in the first place. Others prevent the virus from copying its genetic material once inside. Still others stop newly made virus particles from assembling or breaking free to infect neighboring cells. Because each virus has a unique life cycle, antivirals tend to be narrowly targeted, which is why there’s no single “antiviral pill” the way there are broad-spectrum antibiotics.
Flu Antivirals
Influenza antivirals work best when started within 48 hours of your first symptoms. The most commonly prescribed is oseltamivir (Tamiflu), taken as a pill twice daily for five days. Zanamivir (Relenza) is an inhaled version that works the same way, blocking the enzyme the flu virus uses to spread from cell to cell. Peramivir (Rapivab) is given as a single intravenous dose, typically in a clinic or hospital.
A newer option, baloxavir (Xofluza), requires only a single oral dose. In clinical trials, that single dose shortened symptoms about as effectively as a full five-day course of oseltamivir. Baloxavir works by a different mechanism, targeting the machinery the virus uses to read its own genetic instructions, which makes it a useful alternative when resistance to older flu drugs is a concern.
HIV Antiretrovirals
More than 30 antiretroviral drugs across nine different classes are approved for treating HIV. Most people start a combination regimen of two or three drugs from different classes, taken together to suppress the virus from multiple angles and reduce the risk of resistance. The most commonly recommended starting regimens today center on integrase inhibitors like bictegravir and dolutegravir, which block the virus from inserting its genetic code into your DNA. These are typically paired with two drugs from the older but still essential class of reverse transcriptase inhibitors, such as tenofovir and emtricitabine, which prevent the virus from copying itself.
Other classes include protease inhibitors like darunavir, which stop the virus from assembling into mature, infectious particles, and non-nucleoside reverse transcriptase inhibitors like doravirine and rilpivirine. Many of these come packaged as a single daily pill containing two or three drugs, which simplifies treatment significantly. With consistent use, modern HIV regimens can suppress the virus to undetectable levels, meaning it can’t be transmitted sexually and the immune system can recover.
Herpes and Shingles Medications
Herpes simplex (which causes cold sores and genital herpes) and varicella-zoster (which causes chickenpox and shingles) are closely related viruses, and the same core medications treat both. Acyclovir is the oldest and most widely used. It’s available as a pill, an intravenous formulation for severe cases like herpes encephalitis, and a topical cream. Valacyclovir is essentially a better-absorbed version of acyclovir, meaning you can take fewer pills per day for the same effect.
Famciclovir is a third option, approved for shingles treatment and for treating and suppressing genital herpes outbreaks. Penciclovir, its active form, is also available as a topical cream for cold sores on the lips. None of these drugs cure herpes viruses, which remain dormant in nerve cells for life. Instead, they shorten outbreaks, reduce severity, and when taken daily as suppressive therapy, dramatically lower the frequency of recurrences and the chance of passing the virus to a partner.
Hepatitis C Antivirals
Hepatitis C treatment is one of modern medicine’s biggest success stories. Direct-acting antivirals cure more than 95% of people with hepatitis C in just 8 to 12 weeks of oral treatment. These drugs target three different viral proteins: the protease the virus needs to process its building blocks, the polymerase it uses to copy its RNA, and a structural protein called NS5A that helps the virus replicate and assemble.
Current regimens combine drugs from two or three of these classes into a single daily pill. Treatment is well tolerated compared to older interferon-based therapies, which caused flu-like symptoms for months. After finishing the course, you’re considered cured if the virus is undetectable in your blood 12 weeks later. These treatments are approved for adults and children over age 3.
COVID-19 Antivirals
Two oral antivirals are authorized for treating mild to moderate COVID-19 in people at higher risk of severe illness. Paxlovid (nirmatrelvir with ritonavir) is approved for adults and children 12 and older. Molnupiravir (Lagevrio) is authorized for adults only. Both must be started within five days of symptom onset and are taken at home as pills.
Paxlovid works by blocking the protease enzyme SARS-CoV-2 uses to process its proteins into functional pieces. Molnupiravir takes a different approach, introducing errors into the virus’s genetic code as it tries to copy itself, eventually crippling its ability to reproduce. Both are most effective when started as early as possible after symptoms begin.
Topical and Eye Antivirals
Not all antivirals come as pills. Docosanol (Abreva) is an over-the-counter cream for cold sores on the lips. For eye infections caused by herpes simplex, trifluridine eye drops are a first-line treatment in the United States, typically applied four to eight times daily. Topical acyclovir and ganciclovir gel are also used for herpes infections of the cornea. When deeper layers of the eye are involved, topical steroids are often combined with an antiviral to control both inflammation and viral activity.
Common Side Effects
Side effects vary widely depending on the drug and how long you take it. Flu antivirals like oseltamivir commonly cause nausea and, in rare cases, have been linked to neuropsychiatric symptoms including confusion and vivid dreams. Acyclovir and valacyclovir are generally well tolerated at standard doses, though at high doses or in people with kidney problems, they can occasionally cause tremor, confusion, or hallucinations. Hepatitis C direct-acting antivirals tend to cause mild side effects like headache, fatigue, and nausea. Among HIV drugs, older non-nucleoside reverse transcriptase inhibitors like efavirenz are known for sleep disturbances and vivid nightmares, though newer drugs in the same class cause these problems far less often.
Why Resistance Matters
Viruses mutate rapidly, and when antiviral treatment goes on for weeks, months, or years, resistant strains can emerge. This is a particular concern in people with weakened immune systems, where the virus replicates more freely and has more opportunities to develop mutations that dodge the drug. It’s also why HIV is always treated with multiple drugs simultaneously: if the virus develops resistance to one, the others keep working. For chronic hepatitis B, which requires long-term treatment, resistance has historically been a significant problem, often requiring a switch to more potent drugs or adding a second medication. The short treatment courses used for flu or COVID-19 carry a much lower resistance risk, though it’s not zero, especially when treatment starts late or is taken inconsistently.