You can't delete the values in specific rows of a table. You are deleting the entire row (if you want to erase specific fields, you'd need to do an UPDATE). Do this instead:
SqlCommand cmd = new SqlCommand("DELETE FROM AddProduct WHERE DrugName = @DrugName", conn);
cmd.Parameters.AddWithValue("DrugName", comboBox1.SelectedItem);
Here are some other issues with your code:
- When you don't use parameterized queries, you are opening your code up to SQL injection (that's bad).
- You don't need to set the connection on the command if you pass the connection to the command via the constructor.