The phrase brimstone and treacle, also treacle and brimstone, refers to old-fashioned medicinal remedies.

This phrase was notably used by the English novelist Charles Dickens (1812-1870) in the following passages from Chapter 8 of The Life and Adventures of Nicholas Nickleby (Paris: Published by A. and W. Galignani and Co., 1839):

[page 76]:

“Drat the things,” said the lady, opening the cupboard; “I can’t find the school spoon anywhere.”
“Never mind it, my dear,” observed Squeers in a soothing manner; “it’s of no consequence.”
“No consequence, why how you talk!” retorted Mrs. Squeers sharply; “isn’t it brimstone morning?”
“I forgot, my dear,” rejoined Squeers; “yes, it certainly is. We purify the boys’ bloods now and then, Nickleby.”
“Purify fiddlesticks’ ends,” said his lady. “Don’t think, young man, that we go to the expense of flower of brimstone and molasses just to purify them; because if you think we carry on the business in that way, you’ll find yourself mistaken, and so I tell you plainly.”
“My dear,” said Squeers frowning. “Hem!”
“Oh! nonsense,” rejoined Mrs. Squeers. “If the young man comes to be a teacher here, let him understand at once that we don’t want any foolery about the boys. They have the brimstone and treacle, partly because if they hadn’t something or other in the way of medicine they’d be always ailing and giving a world of trouble, and partly because it spoils their appetites and comes cheaper than breakfast and dinner. So it does them good and us good at the same time, and that’s fair enough I’m sure.”

[page 78]:

Mrs. Squeers stood at one of the desks, presiding over an immense basin of brimstone and treacle, of which delicious compound she administered a large instalment to each boy in succession, using for the purpose a common wooden spoon, which might have been originally manufactured for some gigantic top, and which widened every young gentleman’s mouth considerably, they being all obliged, under heavy corporal penalties, to take in the whole of the bowl at a gasp.

The following two texts give insights into how brimstone and treacle (and other medicinal remedies) were concocted and administered in early-19^th-century Britain:

1-: From the column Died, published in The Lincoln, Rutland, and Stamford Mercury (Stamford, Lincolnshire, England) of Friday 16^th November 1810 [Vol. 79, No. 4,156, page 3, column 4]:

A girl about 12 years of age, daughter of Nathan Richardson, of Spilsby, having been entrusted by her mother to compound some treacle and brimstone, unfortunately mixed a quantity of mercury with the ingredients by mistake, and having partaken of it herself, gave some to three or four other children, one of whom died in consequence the next day, and the rest suffered violently from its effects, but are expected to recover.

2-: From an account of a police-court case, published in The Morning Chronicle (London, England) of Saturday 11^th July 1812 [No. 13,472, page 3, column 5]:

Marlborough-street:—Most shocking catastrophe. John Cooper, and Mary his wife, were charged with feloniously infusing a quantity of arsenic into some medicine, and thereby poisoning four of their children, one of whom died, and the others are not expected to survive. The case will be best judged of by an abstract of the evidence. The Prisoners resided in George-street, Norton-street, Mary-le-bone, are housekeepers, and the man a painter.
Mr. Davis, Surgeon, in Carburton-street, stated, that he was called on to attend the Prisoner’s [sic] children on Thursday evening, and he found four of them in a very dangerous state; they were vomiting, and complained of a gnawing pain at their stomachs, the symptoms were such as would occur after taking poison. Witness understood from the mother that she had given them brimstone and treacle, mixed with sulphur and cream of tartar. Purgations, emetics, were administered, but one of the children died whilst the surgeon was preparing; witness sent for the remains of that the children left, and it was analyzed by the next witness. The body has not yet been opened.
Mr. Selwyn, Chemist, in Mary-le-bone-stteet [sic], analyzed the physic, which had been given the children, and he found it contained a quantity of arsenic. A man had called at witness’s shop, on Wednesday, and purchased some sulphur and treacle, and asked what portion of it made a good mixture, he could not state whether it was the Prisoner or not.
Mrs. Harrington, who lodged in the house of the prisoners, stated that Mrs. Cooper had told her on Tuesday she was about to give her children treacle mixed with sulphur and cream of tartar. Witness was in the yard, and Mrs. Cooper produced a paper with a darkish white powder, and asked her whether she thought it was cream of tartar, as she had magnesia in the house and did not know it apart. Witness tasted it, but she gave no other opinion than that she understood cream of tartar was an acid, and that she tasted was gritty and without taste; but she said she was no judge of such things; and in reply Mrs. C. said, perhaps the acid taste was lost from her having had it so long in the house. On Thursday morning witness saw the children sick, and Mrs. C. expressed surprise, as such had never occurred before. Witness went out, and returned at seven in the evening, when the four children were in bed in a very bad state, and she sent for Mr. Davis; but one child, five years old, died before medicine was afforded. The mother was weeping over them. Witness described the prisoners as living comfortably together, and as being most affectionate parents.
Other lodgers in the house and several neighbours corroborated the latter sentence of the last witness, and described the prisoners as the best of parents.
On being called on to speak, the man was deeply afflicted, urged his attachment to his children, and described his wife as the best of women. He had taken the house at Christmas last, and had no idea how the poison came in the house. He was so much overpowered as to prevent him going on frequently, and the wife, who once fell during the inquiry, was most sensibly affected. One of the three surviving children is expected to die, but such is not the case with the others afflicted. The eldest is nine years old, and the youngest four. The Magistrate feelingly expressed his belief that the catastrophe was the effect of accident; but as he was bound to hold the prisoners until a Coroner’s Jury had sat on the body, he committed them home under the care of an officer.

The earliest occurrences that I have found of the phrase brimstone and treacle, also treacle and brimstone, are as follows, in chronological order:

1-: From Family Medicines, in The Family’s Best Companion, in The Instructor: Or, Young Man’s Best Companion (London: Printed for S. Birt, 1746), by George Fisher, Accomptant [page 323]:

Small-Pox.

When warm in Bed, drink mulled Ale with Marygold-Flowers, and sweat a little, to bring them throughly [sic] out; and to keep them from sinking, take Brimstone and Treacle.

2-: From The Gentleman’s and London Magazine: Or, Monthly Chronologer (Dublin: Printed by John Exshaw) of September 1770 [page 582, column 1]:

A Cure for the Piles or Hemorrhoids.

“Take of liquorice root sliced two ounces, senna and glauber-salts, of each two drachms, sal-prunella one drachm; put these ingredients into three pints of water, and boil them gently over a slow fire, for a quarter of an hour; let the water stand still untill [sic] it is cold, then strain it through a linen cloth, and bottle it off for immediate use, drinking a quarter of a pint to open the body, at night going to bed, and rising in the morning (if found necessary) for a month or six weeks. Salt meat, drams and strong drink, must be refrained from.
P. S. The common receipt of flour of brimstone and treacle mixed up together, in equal quantities, and taken going to bed, of the size of a large nutmeg, so as to open the body, has been known to have given immediate relief, and that may be first tried.”

3-: From Anecdotes, published in The British Mercury, Or Annals of History, Politics, Manners, Literature, Arts, etc. (Hamburgh: Printed for the Editor, and Sold in Commission by B. C. Hoffman) of Monday 9^th July 1787 [Vol. 2, No. 2, page 56]:
—here, brimstone and treacle is comically substituted for the phrase brimstone and fire, also fire and brimstone, referring to the threat of damnation, the torments of hell, or divine punishment:

An enthusiast from North Britain, who has lately made a frequent practice of harranguing [sic] the mob upon religious subjects in Holborn, and many other places about the town, a few days ago, in order to explain the nature of a guilty conscience, affirmed, “that he had not the least doubt but that Cain never went to be shaved on a Saturday night without being afraid the barber would cut his throat.” This exciting the visibility of his auditors, he began to expatiate upon the dissolution of the world, &c. and concluded by telling his hearers, “that it was as plain as a pike staff in scripture, that the world would be suddenly destroyed by brimstone and treacle.”

4-: From Critical Observations and Remarks, on A Stable Directory, Or, Modern System of Farriery, of W. Taplin, Surgeon: Addressed to the Author, in a Series of Letters; In which are pointed out His Errors and Ridiculous Absurdities (London: Printed for the Author, 1792), by A. G. Sinclair, M. D. [page 79]:

It appears that you are totally unacquainted with the properties of the SULPHUR; therefore, permit a NORTH-BRITON to explain them to you, as every body knows that he understands them, or at least ought so to do, having frequent use for this super-excellent mineral.
[…]
Thus have I given you a short description of one of the most extraordinary minerals with which we are acquainted; therefore, I beg that you will give me your reasons for adding sulphur to antimony, Æthiop’s mineral and nitre, which in my opinion are not equal to treacle and brimstone.

5-: From Medical Report of the Endeavour Society, a communication on vermifuge medicines, dated November 1807, that one C. Pears addressed to the Editor of The Literary Panorama, published in The Literary Panorama. Being a Review of Books, Magazines of Varieties, and Annual Register (London: Printed by Cox, Son, and Baylis, for C. Taylor) of December 1807 [Vol. 3, page 603, column 1]:

Mercurial and antimonial remedies have long been given as quack medicines, and are now; but the general principle of all is the same. Some use mechanical remedies, as levigated tin, cowage, &c. to destroy them with an armed force as Dr. Saunders says. Hair taken from the head, and cut very small, answers the purpose of the latter (or cowage), and is frequently given in treacle with the same effect. The common family remedy of brimstone and treacle is a very good one, and is generally found equal to the mercurial (and more expensive) one of Æthiops Mineral, where the mercury is rendered inert, by the brimstone with which it is united.