There are many odd things to put down, and, lest who reads them may fancy that I dined too well before I left Bistritz, let me put down my dinner exactly. I dined on what they call "robber steak" […] The wine was Golden Mediasch, which produces a queer sting on the tongue, which is, however, not disagreeable. I had only a couple of glasses of this, and nothing else. (1.17)
Jonathan Harker assures any future readers of his diary that he wasn't drunk the night he traveled to Castle Dracula. He anticipates that some people might assume that he was hallucinating or in an alcohol-induced haze when they read about the "blue lights" and the wolves, so he tells us exactly what he had to eat and drink beforehand.
[…] I fell to at once on an excellent roast chicken. This, with some cheese and a salad and a bottle of old Tokay, of which I had two glasses, was my supper. (2.18)
Again, Harker is careful to tell us exactly what he had to eat and drink—maybe he's anticipating, again, that future readers of his diary might assume that he was hallucinating the strange things he witnessed at Castle Dracula.
Dr. John Seward
If I don't sleep at once, chloral, the modern Morpheus—C2HCl3O.H2O! I should be careful not to let it grow into a habit. No I shall take none tonight! I have thought of Lucy, and I shall not dishonor her by mixing the two. (8.35)
Chloral, or chloral hydrate, is a sedative that was originally used to treat insomnia, and sometimes used as an anesthetic. It's only mildly addictive, but it was still abused and misprescribed a lot in the late 19th century. Nowadays it's illegal in the US without a prescription. In the late 19th century, though, it wasn't a controlled substance, and doctors, like Jack Seward, could just dose themselves if they had trouble sleeping. No wonder addiction was so common!
[The servants] lay helpless on the floor, breathing heavily. The decanter of sherry was on the table half full, but there was a queer, acrid smell about. I was suspicious, and examined the decanter. It smelt of laudanum, and looking on the sideboard, I found that the bottle which mother's doctor uses for her—oh! did use—was empty. (11.64)
You know, things like this are bound to happen if you leave laudanum (a mixture of opium and alcohol) lying around the house. Laudanum was as common as aspirin in the 19th century. It was used to treat everything from nerves to stomach pain to headaches to insomnia. You could buy it at the corner drug store.
There was no need to think them dead, for their stertorous breathing and the acrid smell of laudanum in the room left no doubt as to their condition. (12.6)
Dr. Seward and Van Helsing, as doctors, recognize the smell of laudanum immediately.
Mina Murray Harker
[…] I asked Dr Seward to give me a little opiate of some kind, as I had not slept well the night before. He very kindly made me up a sleeping draught, which he gave to me, telling me that it would do me no harm, as it was very mild. (19.40)
Even Mina—virtuous, innocent Mina—asks to take some kind of opiate when she has trouble sleeping.