I have been using Effudex on my forehead for 16 days. My question is how do I know when to stop?

Stay out of the sunlight !!!

"Efudex Side Effects:

Important things to remember about Efudex side effects:

•Most people do not experience all of the Efudex side effects listed.
•Efudex side effects are often predictable in terms of their onset and duration.
•Efudex side effects are almost always reversible and will go away after treatment is complete.
•There are many options to help minimize or prevent Efudex side effects.
•There is no relationship between the presence or severity of Efudex side effects and the effectiveness of the medication.
The following Efudex side effects are common (occurring in greater than 30%) for patients taking Efudex:

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•Pain, itching, burning, irritation, inflammation, dryness, swelling, tenderness at the site of application. This will heal once the treatment is complete.
These Efudex side effects are less common side effects (occurring in about 10-29%) of patients receiving Efudex:

•Discoloration of the skin or scarring
•Increased sensitivity to sunlight"

http://www.chemocare.com/bio/efudex.asp