If early drafts of We Have Always Lived in the Castle survive, they are doubtless with Jackson's papers at the Library of Congress in Washington DC. In their absence, however, we can seek some answers in Ruth Franklin's biography of Jackson, A Rather Haunted Life.

Franklin relates that much of the original draft of Castle was "lost or destroyed", but that some of the original plot can be pieced together from the letters that Jackson sent to her close friend, Jeanne Beatty, while in the throes of composition. Checking The Letters of Shirley Jackson reveals that initially (March 1960) Jackson originally considered using mushrooms as the source of poison:

my big problem now is how to kill him; after reading ten million mystery stories i still don’t know a good way. i want something highly suspicious but possibly natural, like mushrooms but i don’t really know one end of a mushroom from another. some highly poisonous garden plant (not oleanders i just happened to find that by chance in the children’s encyclopedia) but i read somewhere larkspur.

So although not thinking of strychnine specifically, she was considering poisons derived from plants. Later that year (September) she went on to amplify her remark about larkspur:

larkspur was used, by the way, in a lovely old english mystery whose name i cannot remember. i never had much faith in mushrooms.

This final comment about mushrooms seems to indicate that she was changing her mind, although she did borrow a field guide to mushrooms from her friend Libbie Burke, and:

made careful notes on the poisonous ones: Amanita pantherina, false blusher (“highly poisonous”); Amanita muscaria, fly agaric (“highly poisonous”); Amanita phalloides, death cap (“deadly poisonous”).

Franklin writes that the final form of the murder was actually inspired by the Bravo case:

a famous case that took place in Victorian England, in which a man named Charles Bravo died mysteriously of antimony poisoning. His wife, Florence, was significantly wealthier and insisted on keeping her money in her name; he may, in fact, have accidentally swallowed poison that he intended for her. The couple’s maid might also have been involved. (The case was never solved.)

Arsenic and antimony have quite similar chemical properties, and so this does seem a likely inspiration for form of murder she eventually chose, although I do not see a reference to it in her published letters.

So it seems that Jackson did indeed consider other forms of poison before settling on arsenic. The sources I have available to me, however, make no mention of strychnine, or indeed Jane Toppan. Possibly this information is in Jackson's unpublished papers, or maybe some information garbling has occurred.

If early drafts of We Have Always Lived in the Castle survive, they are doubtless with Jackson's papers at the Library of Congress in Washington DC. In their absence, however, we can seek some answers in Ruth Franklin's biography of Jackson, Shirley Jackson: A Rather Haunted Life¹.

Franklin relates that much of the original draft of Castle was "lost or destroyed", but that some of the original plot can be pieced together from the letters that Jackson sent to her close friend, Jeanne Beatty, while in the throes of composition. Checking The Letters of Shirley Jackson² reveals that initially (March 1960) Jackson originally considered using mushrooms as the source of poison:

my big problem now is how to kill him; after reading ten million mystery stories i still don’t know a good way. i want something highly suspicious but possibly natural, like mushrooms but i don’t really know one end of a mushroom from another. some highly poisonous garden plant (not oleanders i just happened to find that by chance in the children’s encyclopedia) but i read somewhere larkspur.

So although not thinking of strychnine specifically, she was considering poisons derived from plants. Later that year (September) she went on to amplify her remark about larkspur:

larkspur was used, by the way, in a lovely old english mystery whose name i cannot remember. i never had much faith in mushrooms.

This final comment about mushrooms seems to indicate that she was changing her mind, although she did borrow a field guide to mushrooms from her friend Libbie Burke, and:

made careful notes on the poisonous ones: Amanita pantherina, false blusher (“highly poisonous”); Amanita muscaria, fly agaric (“highly poisonous”); Amanita phalloides, death cap (“deadly poisonous”).

Franklin writes that the final form of the murder was actually inspired by the Bravo case:

a famous case that took place in Victorian England, in which a man named Charles Bravo died mysteriously of antimony poisoning. His wife, Florence, was significantly wealthier and insisted on keeping her money in her name; he may, in fact, have accidentally swallowed poison that he intended for her. The couple’s maid might also have been involved. (The case was never solved.)

Arsenic and antimony have quite similar chemical properties, and so this does seem a likely inspiration for form of murder she eventually chose, although I do not see a reference to it in her published letters.

So it seems that Jackson did indeed consider other forms of poison before settling on arsenic. The sources I have available to me, however, make no mention of strychnine, or indeed Jane Toppan. Possibly this information is in Jackson's unpublished papers, or maybe some information garbling has occurred.

1. Shirley Jackson: A Rather Haunted Life, Ruth Franklin (2016).

2. The Letters of Shirley Jackson, Shirley Jackson, Laurence Jackson Hyman (Editor), Bernice M. Murphy (Contributor), (2021).