Is this the series where...(Possible Spoilers)

I'm trying to think of a drama series I saw years ago where a young Arthur Conan Doyle discovers that a serial killer operating at the time is his friend. His friend has been poisoning people and he even mentions witnessing his friend pass a pill to a woman they were with.

I may be wrong on the series focussing on a young Arthur Conan Doyle. Mostly just remember the show was a fictionalised story about how an author got the idea for a famous character/series.

Any help would be great.