Here's my 2-second search on fluorouracil, from Wikipedia:

http://en.wikipedia.org/wiki/Fluorouracil
The chemotherapy agent 5-FU (fluorouracil), which has been in use against cancer for about 40 years, acts in several ways, but principally as a thymidylate synthase inhibitor. Interrupting the action of this enzyme blocks synthesis of the pyrimidine thymidine, which is a nucleotide required for DNA replication. Thymidylate synthase methylates deoxyuridine monophosphate (dUMP) into thymidine monophosphate (dTMP). Facing a scarcity of dTMP, rapidly dividing cancerous cells undergo cell death via thymineless death[1].

Like many anti-cancer drugs, 5-FU's effects are felt system wide but fall most heavily upon rapidly dividing cells that make heavy use of their nucleotide synthesis machinery, such as cancer cells (other parts of the body with rapidly dividing cells include the cells lining the digestive tract).

Some of its principal uses are in colorectal cancer, and pancreatic cancer, in which it has been the established form of chemotherapy for decades (platinum-containing drugs approved for human use in the US since 1978 are also very well established). It is also sometimes used in the treatment of inflammatory breast cancer, an especially aggressive form of breast cancer.

5-FU is also used in ophthalmic surgery, specifically to augment trabeculectomy (an operation performed to lower the intraocular pressure in patients with glaucoma) in patients deemed to be at high risk for failure. 5-FU acts as an anti-scarring agent in this regard, since excessive scarring at the trabeculectomy site is the main cause for failure of the surgery.

Fluorouracil can be used topically (as a cream) for treating actinic (solar) keratoses and some types of basal cell carcinomas of the skin. It is often referred to by its trade names Efudex, Carac or Fluoroplex.

Due to Fluorouracil's toxicity and the fact that it can be manufactured using the same reaction as uracil, its precursor, 5-Fluoroorotic Acid, is commonly used in laboratories to screen against organisms capable of synthesizing uracil.

It is a key component in Tegafur-uracil.

If you go to the "side effects" section, it sounds rather intense. this seems like a strong drug for something which is supposedly "pre-cancerous."

Regardless, I lately use sunscreen by Mychelle. It's SPF 25 I believe, and available at places like Whole Foods or Vitamin Cottage.

Good luck to you.