Fluorouracil Cream Is Chemotherapy—Here’s How It Works

Fluorouracil cream, most commonly sold under the brand name Efudex, is a genuine chemotherapy drug applied directly to the skin. The same active ingredient, 5-fluorouracil (5-FU), is used intravenously to treat colon, breast, and other internal cancers.¹ When your dermatologist prescribes it as a cream, you are rubbing a cancer-killing drug into your skin, but because the delivery is topical rather than systemic, the experience and the risks bear almost no resemblance to what most people picture when they hear “chemotherapy.”

How 5-Fluorouracil Kills Abnormal Cells

5-FU belongs to a class of drugs called antimetabolites. Your cells need to copy their DNA in order to divide, and that copying process relies on building blocks called nucleotides. 5-FU is structurally similar enough to one of those building blocks that cells absorb it and try to use it. Once inside, the drug gums up the machinery responsible for making new DNA. Cells that are dividing rapidly, like precancerous or cancerous skin cells, take up more of the drug and are hit hardest. Normal, slower-dividing skin cells are affected too, but to a far lesser degree.

Research has also shown that 5-FU does not just damage DNA. It triggers what scientists call an RNA damage response, which pushes affected cells toward programmed death.² In plain terms, the drug attacks abnormal cells on multiple fronts at once: it sabotages their ability to replicate and simultaneously activates their self-destruct sequence. That dual action is part of why it works so well against sun-damaged skin.

What Fluorouracil Cream Treats

The primary use of topical 5-FU is actinic keratosis, those rough, scaly patches that develop on sun-exposed skin after years of ultraviolet damage. Actinic keratoses are considered precancerous because a small percentage of them can evolve into squamous cell carcinoma if left untreated. A single-center study of 150 patients treated with 4% fluorouracil cream found that about 92% achieved complete clearance of all lesions at four months.³ Those are strong numbers for a cream you apply at home.

Beyond actinic keratoses, fluorouracil cream is also used for superficial basal cell carcinoma, the most common skin cancer. In a randomized trial comparing three treatments for superficial basal cell carcinoma, roughly 80% of patients treated with 5% fluorouracil cream were tumor-free at 12 months.⁴ And in studies of Bowen’s disease (squamous cell carcinoma in situ), cryosurgery combined with immediate fluorouracil application achieved a clearance rate of 90% at six months.⁵ The cream’s versatility across these three conditions is a big part of why dermatologists reach for it so often.

The “Field Treatment” Advantage

One of the things that makes fluorouracil cream unique is that it treats an entire area of skin rather than individual spots. Dermatologists call this “field treatment” or “field-directed therapy.” The concept behind it is straightforward: if you have several actinic keratoses on your forehead, there are almost certainly additional patches of sun damage that are not yet visible to the naked eye. Fluorouracil cream treats the whole field, reaching those subclinical lesions before they surface.

This matters for long-term outcomes. In a Veterans Affairs study, patients who used topical fluorouracil had significantly fewer squamous cell carcinoma treatment encounters in the following year compared with those in a control group.⁶ Treating the whole field, not just the lesions you can see, appears to reduce the number of future cancers that develop in that area. Spot treatments like cryotherapy (freezing individual lesions with liquid nitrogen) cannot offer this because they only hit the lesions your doctor can identify at the time of the visit.

Recurrence rates after fluorouracil treatment are not zero. In the single-center study mentioned earlier, about 11% of patients had lesions return within 12 months, mostly in areas with extensive prior sun damage.³ But given that these patients had chronic, widespread damage, a roughly 89% success rate at one year is considered very good for a home-applied cream.

Why Topical Chemo Does Not Feel Like Systemic Chemo

When people hear “chemotherapy,” they think of nausea, hair loss, immune suppression, and weeks of exhaustion. These side effects come from 5-FU flooding the entire bloodstream. Topical fluorouracil cream is a different story. Studies measuring how much drug actually enters the body after skin application found that absorption into the systemic circulation is minimal.⁷ Most of the drug stays right where you put it, in the top layers of skin.

With the lower-concentration 0.5% cream, the amount of fluorouracil that ends up in the urine is about one-fortieth that of the 5% cream, even though the concentration difference is only tenfold. That suggests the 0.5% formulation is even more targeted to the skin itself.⁷ In practical terms, you will not experience the systemic side effects of intravenous chemotherapy. There is no hair loss, no immune suppression, no bone marrow toxicity. What you will experience is intense local skin irritation, which is an entirely different kind of unpleasant.

What Treatment Actually Feels Like

Nobody enjoys a course of fluorouracil cream. The drug works by triggering an inflammatory reaction in sun-damaged cells, and the treated skin responds with redness, swelling, crusting, and sometimes outright ulceration. The worse your skin looks during treatment, the more damage the cream is finding and destroying. This is by design, but it can be alarming if nobody warned you.

Typical courses run two to four weeks for actinic keratoses, depending on the concentration prescribed. During that time, the treated area may look raw and feel tender or burning. Some patients describe it as the worst sunburn of their life. After you stop applying the cream, the skin takes another two to four weeks to heal. During both phases, the treated area can be visually striking enough that people ask whether you are okay.

The skin irritation can tempt people to quit early, and dermatologists have long worried that poor adherence would undermine the cream’s effectiveness. Surprisingly, research has found that adherence to fluorouracil cream tends to be quite good. One study speculated that the visible reaction actually encourages patients to keep going because the angry skin makes them feel the drug is working.⁸ The adverse effects, though uncomfortable, were mild to moderate in most patients and not bothersome enough to override the motivation to finish treatment.

That said, in elderly patients (who represent a large share of actinic keratosis cases), minimizing irritation matters for quality of life. The 0.5% formulation was developed partly to address this. It requires only once-daily application, and patients report finding it more tolerable than the 5% cream.⁹

0.5% Versus 5% Cream

One of the more counterintuitive findings in dermatology is that the lower-concentration fluorouracil cream performs just as well as, and in some measures better than, the stronger formulation. In a split-face study where patients applied 0.5% cream to one side and 5% cream to the other, both achieved similar total clearance rates of roughly 43%. But the 0.5% cream reduced the absolute number of lesions more dramatically, from an average of about 11 down to 2.5, compared with 10 down to 4 for the 5% cream.¹⁰

The 0.5% cream uses a microsponge delivery system that releases the drug slowly and keeps more of it in the skin rather than letting it pass through into the bloodstream. About 85% of patients in the split-face study preferred the lower-concentration cream, citing less irritation and a simpler dosing schedule.¹¹ The practical takeaway: if your dermatologist offers you a choice, the 0.5% formulation tends to be easier to live with and the results are comparable. But some situations, like superficial basal cell carcinoma, still call for the higher concentration.

How It Stacks Up Against Other Treatments

Fluorouracil cream is not the only option for actinic keratoses. The most common alternatives include imiquimod cream, ingenol mebutate gel, and photodynamic therapy (PDT), which uses a light-activated chemical to destroy abnormal cells. Across head-to-head comparisons, fluorouracil consistently comes out well.

For superficial basal cell carcinoma, a large randomized trial found that fluorouracil cream achieved about 80% tumor-free status at one year, roughly in line with PDT (about 73%) and not far behind imiquimod (about 83%).⁴ For actinic keratoses, fluorouracil cream and PDT produce clinically similar responses, though PDT tends to cause less aggressive skin reactions during treatment.¹²

Where fluorouracil cream really pulls ahead is on cost. A Dutch cost-effectiveness analysis found that the total mean costs for a 5-FU treatment course were €433, compared with €728 for imiquimod, €775 for ingenol mebutate, and €1,621 for photodynamic therapy. Fluorouracil was both more effective and less expensive than all three comparators at 12 months, the rare treatment that dominates on both fronts.¹³ An independent UK appraisal echoed this, concluding that 5% fluorouracil cream is the best first-line treatment for actinic keratosis on both effectiveness and cost grounds.¹⁴

Newer Combination Approaches

Researchers have been experimenting with pairing fluorouracil with other agents to improve outcomes or shorten treatment. One promising combination adds calcipotriol, a synthetic form of vitamin D. The idea is that calcipotriol stimulates the immune system’s response against abnormal cells while fluorouracil does the direct killing. Early data suggest the combination can reduce both the duration of treatment and the severity of skin irritation compared with fluorouracil alone.¹⁵ This combination is not yet a standard of care, but it is generating interest as a way to make an already effective treatment more tolerable.

Nanoemulsion formulations are another area of development. By engineering 5-FU into nanoparticles, researchers aim to improve how deeply and evenly the drug penetrates the skin, which could enhance its effectiveness against deeper lesions without increasing the amount that reaches the bloodstream.¹⁶

The DPYD Question and Who Should Be Cautious

One safety concern that sometimes comes up is DPYD deficiency. DPYD is a gene that encodes an enzyme your body uses to break down fluorouracil. People who carry certain variants of this gene process the drug more slowly, which can lead to dangerous toxicity when 5-FU is given intravenously. Genetic testing before IV chemotherapy with fluorouracil is now recommended in many countries.

But does this apply to the cream? Research specifically examined this question and found that patients carrying a single DPYD variant allele did not have a significantly higher risk of toxicity from topical fluorouracil. None of the patients in the study experienced severe toxicity.¹⁷ The researchers concluded that there is limited benefit to genetic testing before prescribing topical 5-FU. The one exception: people with complete DPD deficiency (meaning both copies of the gene are nonfunctional) should still avoid the cream, because the risk of severe toxicity in that rare group remains unknown. If you know you have complete DPD deficiency, tell your dermatologist before starting treatment.

Uses Beyond Skin Cancer and Precancer

Fluorouracil has found a second life in treating keloids and hypertrophic scars. These are raised, sometimes painful scars where the wound-healing process went into overdrive and produced too much collagen. Injecting 5-FU directly into keloids disrupts the overactive fibroblasts that are building excess scar tissue. A systematic review of intralesional 5-FU for keloids found that about 73% of treated keloids showed at least 25% improvement, and about 67% showed at least 50% improvement. The relapse rate was around 16% at roughly six months after treatment.¹⁸

When 5-FU injections are combined with steroid injections (triamcinolone acetonide), the results improve further, with up to 96% of patients achieving good to excellent scar improvement in some studies.¹⁹ For hypertrophic scars specifically, 5-FU improved pliability, thickness, and height of the scar tissue.²⁰ These uses are considered off-label, but they are well-established in dermatology practice and not fringe at all.

Fluorouracil cream has also been tried on dysplastic melanocytic nevi, which are atypical moles that carry an elevated risk of melanoma. In an early study, six dysplastic nevi treated with 5% fluorouracil cream responded with inflammation and ulceration, then disappeared entirely, while control moles were unaffected.²¹ This application has not become mainstream, but it illustrates how selectively the cream targets abnormal cells while leaving normal tissue alone.

Keeping Pets Safe During Treatment

There is one safety issue with fluorouracil cream that has nothing to do with human health and everything to do with household pets. Fluorouracil is extremely toxic to dogs and cats. A case report documented a young Labrador-poodle mix that was rushed to a veterinary hospital after ingesting topical 5-FU solution.²² Dogs lack the enzymatic pathways needed to metabolize the drug, so even small amounts can cause seizures, bone marrow failure, and death. If you are using fluorouracil cream at home and have pets, keep the tube stored securely, wash your hands thoroughly after every application, cover the treated area so animals cannot lick it, and dispose of any used gloves or tissues where pets cannot reach them. This is not an overreaction. Veterinary poisoning cases from fluorouracil have been well documented, and the outcome is often fatal for the animal.