a multiple selection variable calculates frequency counts

Text parsing like this can be difficult without a specific, reproducible example. However, it sounds as though your data frame looks something like this:

df
#>    ID medication gender
#> 1   1          9      f
#> 2   2      2;1;3      m
#> 3   3        6;2      d
#> 4   4          3      f
#> 5   5    7;8;7;1      f
#> 6   6    6;9;4;6      m
#> 7   7          9      d
#> 8   8      8;6;3      f
#> 9   9        9;7      d
#> 10 10        8;6      m

In which case, the pedestrian way to get your result in base R would be something like this:

meds <- lapply(split(df, df$gender), 
               function(x) unlist(strsplit(x$medication, ";\\s?")))

genders <- rep(c("d", "f", "m"), times = lengths(meds))

table(gender = genders, medication = unlist(meds))
#>       medication
#> gender 1 2 3 4 5 6 7 8 9 10
#>      d 0 1 0 0 0 1 1 0 2  0
#>      f 1 0 2 0 0 1 2 2 1  0
#>      m 1 1 1 1 0 3 0 1 1  0

Reproducible data

set.seed(2)

medication <- sapply(rpois(10, 2), function(x) {
  if(x == 0) x <- 1
  x <- sample(1:10, x, TRUE)
  paste(x, collapse = ";")
})

gender <- sample(c("m", "f", "d"), 10, TRUE, prob = c(2, 2, 1))

df <- data.frame(ID = 1:10, medication = medication, gender = gender)

^{Created on 2022-02-06 by the reprex package (v2.0.1)}