Carac cream on lip

I am using carac cream on my lip for what my doctor told me was for a pre-cancerous condition. I am using it on the entire bottom lip, and I worry that I about the cream getting into my mouth or on my tongue. How harmful is it if this happens?  I read that some people use Vasoline to provide moisture. Is this a possible way to keep the cream on my lip and not in my mouth?

 I am not supposed to be exposed to direct sunlight which is difficult for me to do sometimes. Can I use a sunblock (Neutogena Ultra Sheer) or Burt's Bee Lifegaurd Choice (Weatherproofing)?

 I  hope those of you with experience and expertise can answer my questions. I am in Korea working and it is difficult to contact my doctor. Thank you.

 3 weeks ago I had a Ak surg. removed from under my nose thru to my upper lip all the way to the bottom of the upper lip. My dr told me my entire lip both upper and lower are covered in them and gave me Carac to use. I will start this weekend. After reading the other post on how it dried the face out, i'm worried how it will do on the very thin lip skin.