Derma smoothe on ethnic hair?

Today, I went to the dermatologist for dandruff. I don't have psoriasis or anything. My hair gets flaky 3-4 days after I wash it. She told me to get selsun blue or vizora and she prescribed me derma smoothe. It says apply 1-2 times a week. However, I'm black LOL so should I only apply it once a week when I wash my hair considering we can't wash our hair as often.?? also if anyone suffers with bad dandruff and has used derma smoothe, is it ok to wash it out with selsun blue since it's a medicated shampoo.? Thanks in advanced.!!