Now there is a possibility to taxate used medicine directly through the taxation window using PAGE DOWN.
When the first product is taxated, doubleclick on the line and get into the "Insert treatment" window.

Highlight the medicine field, delete the text and search for the new product that is going to be invoiced.

When you find it, it's very important to press PAGE DOWN as this will add the product to the taxation instead of changing the original product to the new.