As the person who tagged that question health-policy and thereby created the tag, I think they should be on topic.
A related, although not identical, issue was discussed in this answer about "practice of medicine" questions. To me, health policy issues are similarly an area that may be education for professionals in the field.
On the other hand, this is not a site about law or politics. The question cited by the OP (which, as it happens, was migrated here by the moderators at Law.SE) struck me as on topic specifically because answering it would, ideally, entail discussion of pharmacology and aspects of risk/benefit ratio. These parameters are clearly on topic here.
This question, then, is at the intersection of policy and medicine. Although purely legal or political questions would be better suited to another SE site, I think questions about policy that directly engage issues of physiology, pathology, and/or pharmacology, should be allowed, tagged health-policy. Insofar as the answer may touch on purely political considerations, these should not be the focus here. Perhaps we could revise that question to ask:
What were the risks of phenmetrazine that caused it to be banned in the UK?
Note: It may be argued that policy varies from country to country, and an answer on health.SE can not be expected to survey the globe to discuss all extant policies on a matter. I agree. This question was answerable specifically because it was specific. It asked about a specific policy in a specific country, the reasoning behind which is expected to be grounded in physiology and pharmacology.