The closest I've found to a definitive statement is this one:
Carroll’s diaries, which are on display at the exhibit, make no mention of drugs. We know that he occasionally enjoyed a glass of sherry and may have taken opiate-infused drug Laudanum (which was readily available to everyone in the 1860s). Other than that, there’s nothing connecting Alice and drugs.
The Alice in Wonderland fansite says much the same in their FAQ:
No evidence has ever been found that linked Carroll to recreational drug use. Even in his extensive diaries, Carroll has never made any reference to the use of drugs.
There is however one part in the book that may describe the use of drugs: the hookah smoking Caterpillar who advises Alice to eat from the mushroom. With the story Carroll made fun of all aspects of society, and it may be possible that he was reflecting the age with this part: In the Victorian era there were no drug laws like we know them now. Opium, cocaine, and laudanum (a painkiller that contained opium) were used for medicinal purposes, and could be obtained from a pharmacist (mind that LSD was not yet invented at the time!). So in Carroll’s days it was not uncommon to experience the effect of being ‘high’, whether or not accidentally.
They then note that the Caterpillar wasn't even in the original tale told by Alice, but added later when he was assembling the book for publishing.
Just to cover all of the bases, some of the historical record of his diary and correspondence is no longer available to reference:
.... Dodgson’s diaries, which were published in abridged form in 1954 and in full, with Wakeling’s annotations, beginning in 1993. But even they are an imperfect source. Four of the 13 volumes are missing — as are the pages covering late June 1863, when his break with the Liddells occurred. A Dodgson descendant apparently cut them out after the writer died.
That said, what has survived does not support the idea that the books were written under the influence of drugs nor that they were about drugs.
