Is it permissible to work in pharmacies in the United Kingdom given the possibility that some medications may contain unlawful ingredients?

The default ruling is that selling medicines is permissible unless something is known to be prohibited, and nothing becomes prohibited merely by doubt. It is permissible to sell medicines that contain a minute proportion of alcohol or prohibited derivatives (such as derivatives from carrion or swine) if the alcohol content is so negligible that large quantities of it do not intoxicate, or if it has undergone a chemical transformation that changes its nature into a pure substance. This ruling applies to medicines prescribed by a trustworthy physician, and when no alternative medicine free of these substances is readily available. However, if the medicine intoxicates in large quantities or contains swine fat without undergoing a transformation, then selling it is not permissible. The default ruling for working in pharmacies is permissibility, and one must avoid selling prohibited items when their prohibition is confirmed.