No, you couldn't use warehouse in this context. If it's a room where the pharmacy keeps their supplies, you could call it a store room or a stock room. 'House' means building, and a warehouse is a large, independent building. Products would typically be transported from the factory to a warehouse, where they'd be stored in large quantities before being shipped out to retailers or customers. A large retailer might have its own warehouse, but this would be a separate building from the retail premises.

I wouldn't refer to anything as a "warehouse" that couldn't also be repurposed as a barn. A warehouse is a large and spacious building where you can store anything you'd like, from animals to airplanes. An area where you store things for your business could be a storeroom (a place where you keep things in storage), a stockroom (a place where you keep materials you have in stock), or just your backroom (another way to express that you have more things stored in the back of your business). I hope this was helpful!

I mean, I guess so. I've been born in England and lived here my whole life but I've never had to refer to that place specifically, but I guess they'd definitely know what you mean by warehouse