You have written a very nice piece. It is easy to understand. Well done. "It is about a man ..... to treat for ...." You need to change 'to treat for.' You could say he went to the library to learn how to CURE a slight sickness. Or, to TREAT a slight sickness (not 'for'). The use of 'accidentally' puzzles me. He went there DELIBERATELY. There was nothing accidental about it. The doctor WROTE a prescription, or PRESCRIBED something. It is the chemist who will make up the prescription (get/mix/make the medicine/count out the pills/etc.) So the man went to the chemist/pharmacy where the chemist said the PRESCRIPTION was .... (not 'recipe') UNLESS the AUTHOR HIMSELF USED THE WORD 'recipe.' We have 'the recipe for a good life' (an expression). I don't know if YOU chose the word recipe, or the author did. I'm not sure what a drugstore is. We don't have them here. The 3 men and Montgomery (wasn't that the name of the dog?) would have had only the chemist to go to. I believe 'drugstores' often sell far more than medicines and cosmetics. But you need to ask an American! Directions FOR a healthy lifestyle... (not OF) A few more commas might be useful... you can decide where you need them. (!)

I think the grammar is fine. I would add a few more commas: "The chemist, however, said he..." "Actually, he found out..." This is a small item, and probably it depends on how the reader, or speaker, would read or speak the sentence. Your style communicates well. That is the purpose of language.