Antiviral medications are drugs that fight viral infections by blocking specific steps in a virus’s life cycle. Unlike antibiotics, which kill bacteria, antivirals work by preventing a virus from entering cells, copying its genetic material, or releasing new virus particles. Dozens of antivirals are available today, targeting everything from the flu and herpes to HIV, hepatitis C, and COVID-19.
How Antivirals Work
Every virus follows a predictable sequence: it attaches to a human cell, penetrates it, sheds its outer coat, replicates its genetic material, assembles new copies of itself, and releases those copies to infect more cells. Antiviral drugs are designed to interrupt one of these steps so that no new virus is produced. Some block the virus from latching onto a cell in the first place. Others shut down the machinery it uses to copy its DNA or RNA. Still others prevent newly assembled virus particles from breaking free.
This targeted approach is why most antivirals only work against one virus or a small family of related viruses. A flu antiviral won’t help with herpes, and an HIV drug won’t treat hepatitis C. It also explains why timing matters: antivirals are most effective when the virus is still actively replicating, which is usually early in an infection.
Antivirals for Influenza
Four FDA-approved antiviral drugs are currently recommended for treating the flu. The most widely prescribed is oseltamivir (Tamiflu), taken as a pill or liquid twice daily for five days. Zanamivir (Relenza) is an inhaled powder, peramivir (Rapivab) is given as a one-time IV infusion by a healthcare provider, and baloxavir (Xofluza) is a single-dose pill taken by mouth.
All four work best when started within 48 hours of the first symptoms. Within that window, they can shorten the duration of fever and illness and reduce the risk of complications like pneumonia and ear infections in children. Even when started after 48 hours, there’s some evidence of benefit: one trial in children found that starting oseltamivir at 72 hours still reduced symptoms by about a day compared to no treatment. Baloxavir performs comparably to oseltamivir in clinical trials, with the added convenience of being just one pill.
Side effects from flu antivirals are generally mild. Oseltamivir causes nausea in roughly 10% of adults and vomiting in about 9%, compared to 6% and 3% with placebo. Zanamivir’s side effect profile in trials was nearly identical to placebo, with headache, dizziness, nausea, and cough each occurring in fewer than 5% of patients.
Antivirals for Herpes and Shingles
Three closely related drugs treat infections caused by herpes simplex virus (which causes cold sores and genital herpes) and varicella-zoster virus (which causes chickenpox and shingles): acyclovir, valacyclovir, and famciclovir. All three are taken as pills, though acyclovir also comes in IV and topical forms.
Valacyclovir is essentially a modified version of acyclovir that the body absorbs more efficiently, so you take it less often (twice a day instead of three to five times). Famciclovir similarly has good absorption and is dosed three times daily for initial outbreaks. A first episode of genital herpes is typically treated for 7 to 10 days.
These medications control symptoms and speed healing, but they don’t eliminate the virus. Herpes viruses hide in nerve cells in a dormant state that no current drug can reach. Once you stop taking the medication, outbreaks can return. For people with frequent recurrences, daily suppressive therapy with one of these drugs reduces flare-ups significantly.
Antivirals for HIV
HIV treatment has transformed from a death sentence into a manageable chronic condition, largely because of antiretroviral therapy. Eight distinct classes of HIV drugs exist, each targeting a different moment in the virus’s life cycle. Some block the virus from entering immune cells. Others prevent it from converting its RNA into DNA, integrating that DNA into human chromosomes, or assembling new viral particles.
Modern HIV treatment typically combines two or three drugs from different classes into a single daily pill. This multi-drug approach is critical because HIV mutates rapidly, and attacking it at several points simultaneously makes it far harder for resistant strains to emerge. When taken consistently, these regimens can reduce the amount of virus in the blood to undetectable levels, which both preserves immune function and prevents transmission to others.
Antivirals for Hepatitis C
Hepatitis C treatment is one of the biggest success stories in antiviral medicine. A class of drugs called direct-acting antivirals, introduced in the mid-2010s, can cure the infection in the vast majority of patients. “Cure” here means the virus is completely eliminated from the body, not just suppressed.
These medications are combination pills that attack the hepatitis C virus at multiple stages of its replication. Several options are available, including sofosbuvir/ledipasvir (Harvoni), sofosbuvir/velpatasvir (Epclusa), and glecaprevir/pibrentasvir (Mavyret). Treatment courses run 8 to 12 weeks depending on the specific drug and how much liver damage has occurred. Before these drugs existed, hepatitis C treatment involved months of injections with severe side effects and cure rates well below 50%.
Antivirals for COVID-19
Paxlovid became the first widely used oral antiviral for COVID-19. It contains two components: one that directly blocks the virus from replicating, and a booster drug (originally developed for HIV) that slows the liver’s breakdown of the first drug so it stays active longer. The standard course is three pills twice a day for five days, totaling 30 pills. A reduced dose is available for people with moderate kidney impairment.
Paxlovid is approved for anyone 12 and older who is at high risk of severe COVID-19. It needs to be started within the first five days of symptoms to be effective, and its primary benefit is reducing the chance of hospitalization. One important consideration is that the booster component interacts with a long list of other medications, including certain blood thinners, heart drugs, and cholesterol medications. Your pharmacist will review your full medication list before dispensing it.
Antivirals for RSV
Respiratory syncytial virus, a common cause of serious lung infections in infants and young children, has limited antiviral options compared to other viruses. Aerosolized ribavirin is approved for treating hospitalized infants and young children with severe RSV lung infections, though in practice it’s used mainly when the infection could be life-threatening, such as in bone marrow transplant patients.
For prevention rather than treatment, palivizumab is given as monthly injections during RSV season to high-risk infants: those born prematurely (at 35 weeks or earlier) or those under two years old with chronic lung disease. Newer preventive options, including long-acting antibody injections and maternal vaccines, have expanded the tools available for RSV in recent years.
Common Side Effects Across Antivirals
Most oral antivirals share a handful of common side effects: nausea, headache, dizziness, and diarrhea. These tend to be mild and resolve on their own. Nausea is particularly common with oseltamivir and some HIV medications, especially in the first few days of treatment.
Kidney function is a recurring consideration. Several antivirals, including acyclovir and some HIV drugs, are cleared through the kidneys, and doses often need to be adjusted for people with reduced kidney function. Drug interactions are another broad concern, particularly with HIV antiretrovirals and Paxlovid, both of which affect how the liver processes other medications. The specific side effect profile varies considerably from one antiviral to another, so what applies to a flu drug won’t necessarily apply to a hepatitis C treatment.