Fluorouracil cream works by mimicking a building block of DNA, tricking fast-growing skin cells into absorbing it, then shutting down their ability to replicate. Because precancerous and cancerous skin cells divide much faster than normal skin cells, the cream selectively destroys them while leaving healthy tissue largely intact. It’s one of the most widely used topical treatments for actinic keratoses (rough, scaly patches caused by sun damage) and superficial basal cell carcinoma.
The Biochemistry Behind the Cream
Your cells need a specific molecule called thymidine to build and repair DNA. An enzyme called thymidylate synthase produces this molecule, and it’s the only way your cells can make it internally. Fluorouracil is an antimetabolite, meaning it’s structurally similar enough to a natural compound that cells absorb it, but different enough to cause problems once inside.
Once fluorouracil enters a cell, it gets converted into active compounds that block thymidylate synthase. Without that enzyme working, the cell can’t produce the thymidine it needs, and DNA replication grinds to a halt. The cell can’t divide and eventually dies. On top of that, some of fluorouracil’s byproducts get directly incorporated into RNA and DNA strands, further disrupting normal cell function and triggering cell death through multiple pathways at once.
Why It Targets Damaged Cells
The cream’s selectivity comes down to one simple principle: speed of division. Precancerous cells in actinic keratoses and superficial basal cell carcinomas are dividing far more rapidly than the normal skin surrounding them. Because fluorouracil disrupts DNA replication, cells that are actively and frequently dividing absorb more of the drug and suffer more damage. Normal skin cells, which divide at a much slower rate, are exposed to the same concentration of drug but take up far less of it and have more time to repair any minor damage.
This is also why treatment makes hidden sun damage visible. Many people are surprised when the cream causes redness and irritation in areas that looked perfectly normal before application. Those patches contained precancerous cells that hadn’t yet become visible lesions, and fluorouracil reveals them by triggering the same inflammatory destruction.
What It Treats
Fluorouracil cream is FDA-approved for two conditions. The primary use is actinic keratosis, sometimes called solar keratosis. These are rough, scaly spots that develop on sun-exposed skin after years of UV damage. Left untreated, a small percentage of actinic keratoses can progress to squamous cell carcinoma.
The second approved use, available only in the 5% concentration (brand name Efudex), is superficial basal cell carcinoma. This applies specifically to patients whose tumors can’t be treated with conventional methods like surgery. It’s not appropriate for deeper or more aggressive forms of skin cancer.
Available Strengths and Formulations
Fluorouracil cream comes in several concentrations under different brand names. Carac is the lowest at 0.5%. Fluoroplex comes as a 1% cream or solution. Tolak is a 4% cream. Efudex, the most widely recognized brand, is available as a 5% cream or solution, as well as a 2% solution. Your prescriber chooses the concentration based on the condition being treated, the location on the body, and how your skin typically tolerates topical medications. Lower concentrations may cause less irritation but often require longer treatment courses.
What to Expect During Treatment
Fluorouracil cream is typically applied once or twice daily to the affected area. Treatment length varies depending on the condition and concentration used, but courses for actinic keratosis commonly run two to four weeks. Superficial basal cell carcinoma may require longer treatment.
The skin reaction follows a fairly predictable pattern. During the first week, you may notice mild redness and tingling at the application sites. By weeks one to two, the treated skin becomes noticeably red, irritated, and begins to peel. This is the drug working, not a sign that something has gone wrong. The reaction can intensify as treatment continues, with some areas developing crusting, swelling, or even open sores in spots where sun damage is most concentrated.
After you stop applying the cream, the inflammatory reaction continues for several more weeks before the skin begins to heal. New, healthy skin gradually replaces the treated areas. Full healing typically takes two to six weeks after the last application, though this varies by person and by how much sun damage was present.
Managing the Skin Reaction
The inflammation caused by fluorouracil is intentional, but it can be uncomfortable. If the reaction becomes severe, you have a few options that don’t compromise results. Reducing application frequency (for example, switching from twice daily to once daily or applying every other day) can dial back the intensity. Applying a low-potency topical steroid alongside fluorouracil, and continuing it for one to two weeks after stopping the cream, can also reduce inflammation without affecting how well the treatment works.
Keeping the treated skin moisturized and protected from sun exposure during and after treatment helps with comfort and healing. The treated areas will be more sensitive to UV light, so sun protection is especially important during this period.
How Much Gets Into Your Bloodstream
When applied to intact skin, fluorouracil is minimally absorbed into the body. This is one of the key advantages of topical treatment over systemic chemotherapy. However, if the skin barrier is compromised (as happens naturally during treatment when the skin becomes raw or ulcerated), absorption can increase dramatically, up to 60% in damaged areas. Even with this increased local absorption, the total amount entering the bloodstream from a typical treatment area remains small compared to intravenous fluorouracil used for internal cancers.
A Rare but Serious Genetic Risk
About 3 to 8% of the population carries variants in a gene called DPYD, which produces the enzyme responsible for breaking down more than 80% of fluorouracil in the body. People with partial deficiency of this enzyme may experience unusually severe reactions. Those with complete deficiency face a risk of serious, potentially fatal toxicity even from topical application, because whatever fluorouracil does get absorbed cannot be properly cleared.
The FDA now requires a boxed warning on fluorouracil products about this risk and recommends genetic testing before starting treatment when possible. If you have a family history of severe reactions to fluorouracil or its related drug capecitabine, this is worth discussing before beginning a course of treatment. For the vast majority of people, though, topical fluorouracil is well-tolerated and the systemic exposure remains low enough that serious complications are rare.