Topical chemotherapy is a cancer treatment applied directly to the skin as a cream or gel, rather than delivered through an IV or taken as a pill. It targets abnormal or cancerous cells on the skin’s surface while leaving the rest of the body largely unaffected. The most common form uses an active ingredient called 5-fluorouracil (5-FU), and it’s primarily prescribed for precancerous growths called actinic keratoses and for superficial basal cell carcinoma, the most common type of skin cancer.
Conditions It Treats
Topical chemotherapy is reserved for skin conditions that sit on or very close to the surface. The two main uses are actinic keratoses, which are rough, scaly patches caused by years of sun exposure that can eventually turn into skin cancer, and superficial basal cell carcinoma, a slow-growing cancer confined to the top layer of skin. Doctors also sometimes prescribe it for certain warts and superficial squamous cell carcinomas.
The key word is “superficial.” If a skin cancer has grown deeper into tissue or spread to other areas, topical treatment alone won’t reach it. In those cases, surgery, radiation, or systemic chemotherapy are more appropriate.
How It Works on the Skin
The most widely used topical chemotherapy, 5-FU, belongs to a class of drugs called antimetabolites. It kills fast-growing cells by interfering with DNA replication. Once the cream is absorbed into abnormal skin cells, the active ingredient blocks a key enzyme those cells need to copy their DNA. Without that enzyme, the cells can’t divide. The drug also gets mistakenly built into the cells’ DNA and RNA, causing so much internal damage that the cell’s own repair systems become overwhelmed, and the cell dies.
Because cancer cells and precancerous cells divide much faster than normal skin cells, they absorb more of the drug and are hit hardest. Normal, healthy skin in the treated area experiences some irritation but generally recovers once treatment ends.
A different topical option, imiquimod, works through an entirely different pathway. Rather than poisoning cells directly, it activates your immune system at the application site. It triggers the release of signaling molecules that recruit immune cells to the area, essentially alerting your body to attack the abnormal cells. Imiquimod also directly causes cancer cells to self-destruct through a process called apoptosis. It’s approved for actinic keratoses, superficial basal cell carcinoma, and certain genital warts.
Why Side Effects Stay Local
One of the biggest advantages of topical chemotherapy over the IV kind is that virtually none of the drug reaches the rest of your body. Lab studies measuring how much cytotoxic drug passes through human skin found absorption levels so low they were often undetectable, typically less than 0.001% of the applied dose. That’s why topical chemotherapy doesn’t cause the hair loss, nausea, or immune suppression associated with systemic chemotherapy. The side effects are confined to the skin you’re treating.
What the Treatment Looks Like
Treatment with 5-FU typically involves applying a thin layer of cream to the affected area once or twice daily for several weeks, depending on the condition being treated and the concentration prescribed. Actinic keratoses usually require two to four weeks of treatment, while superficial basal cell carcinoma may need longer courses. Imiquimod follows a different schedule, often applied a few times per week rather than daily.
You apply the cream at home, which is a major practical advantage over treatments that require repeated office visits. Your dermatologist will outline exactly how much to use, how often, and for how long. The treated skin should not be covered with airtight bandages unless specifically instructed.
Skin Reactions to Expect
Topical chemotherapy deliberately creates an inflammatory reaction in the treated skin, and this is actually a sign the medication is working. During the first two weeks, the area typically becomes red, swollen, and crusty. Intense itching is common. The skin may blister, peel, or develop shallow sores as abnormal cells are destroyed. This can look alarming, but it’s the expected progression.
Some people develop bacterial skin infections over the treated area during this inflammatory phase, though these are usually short-lived. After you stop applying the cream, the skin enters a healing phase. Redness and irritation gradually fade over the following two to four weeks, and new, healthy skin replaces what was destroyed. The final cosmetic result is generally good, often better than what you’d get from freezing or surgical removal, since the treatment targets damaged cells across an entire area rather than one spot at a time.
During and after treatment, the treated skin becomes significantly more sensitive to sunlight. You’ll need to protect those areas from sun exposure for several weeks to prevent sunburn and further damage.
How Effective Is It?
For actinic keratoses, topical 5-FU has strong evidence behind it. In a clinical study of 150 patients treated with 4% 5-FU cream, 92% achieved complete clearance of their lesions. A head-to-head trial comparing different treatments found clinical clearance rates of 96% for 5-FU, 85% for imiquimod, and 68% for cryotherapy (freezing).
Long-term results tell a slightly different story. At the one-year mark, sustained clearance was 54% for 5-FU compared to 73% for imiquimod and just 28% for cryotherapy. That means some lesions do come back after initial treatment, and repeat courses or combination approaches are sometimes needed. A large meta-analysis pooling data from over 2,200 patients found that 5% 5-FU cream achieved complete patient-level clearance of about 57%, outperforming most other topical options.
These numbers highlight an important distinction: clearing individual lesions is easier than clearing every precancerous spot on a patient’s skin. Your dermatologist may recommend topical chemotherapy as a “field treatment,” covering a broad area of sun-damaged skin to catch both visible and not-yet-visible lesions, followed by monitoring for anything that returns.
How It Compares to Other Options
Topical chemotherapy isn’t the only way to treat surface-level skin cancers and precancerous spots. Cryotherapy (freezing with liquid nitrogen) is quick and works well for isolated lesions, but it treats one spot at a time and has lower long-term clearance rates. Surgical excision is the gold standard for confirmed skin cancers because it allows the tissue to be examined under a microscope, but it leaves scars and isn’t practical for dozens of scattered actinic keratoses.
Topical chemotherapy fills a specific niche: treating widespread precancerous damage across a broad area of skin, or managing superficial cancers in patients who want to avoid surgery. It’s also useful for areas where scarring would be cosmetically undesirable, like the face and scalp. The tradeoff is several weeks of visible skin irritation and discomfort during the treatment period, which some people find difficult to tolerate, especially on the face.
Diclofenac gel is another topical option for actinic keratoses, though it’s considerably less effective. In comparative analyses, it achieved clearance rates well below those of 5-FU or imiquimod, making it a secondary choice typically reserved for people who can’t tolerate stronger treatments.