How can I count the frequency of multiples columns with same categorical variables?

I have five columns and I need to count the frequency of variables together, but I don't know to do this.
rstudio
Each row is a patient and the columns are the combination of the drugs which they use. I need to know, for example, how many times the "Prednisone" appears on these columns, and, thus for all drugs. Can someone help me?
Thank you!

Hi!

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