"The Adulteration Of Malt Liquor hue been most carefully examined by a committee of the House of Commons, and since then by Dr. Hassall, who first had his attention called to the bitter beer of the Burton breweries, which was said to be adulterated with strychnine, in order to obtain a cheap bitter. This has been most completely disproved by several high chemical authorities, and also by Dr. Hassall, at the instigation of the Lancet, all coming to the conclusion that the bitter ale of Messrs. Bass and Co and Allsop and Co is a pure and wholesome beverage, concocted from malt, hops, and water alone. With regard to stout and porter, several seizures of illegal articles to "make up" quantities of them have been made by the Excise, consisting of cocculus indicus, Spanish Juice, grains of paradise, copperas, quassia, etc, so that it has been indisputably proved that the attempt has been made to use these noxious articles in the trade. It is well-known that publicans are very careful how they allow their customers to invade their underground territories in the morning, when the porter, etc, for the next day is preparing; but this is not necessarily a proof of guilt, but rather may he caused by a desire to be free from interruption. Nevertheless, they are universally charged with these tricks, and by general consent it is supposed that something or other is done in the cellar which will not bear the light of day. Dr. Hassall, however, was unable to detect any adulteration in porter or stout, except with water, which is а venial offence, comparatively, and, in addition, a certain portion of salt, which, also, is only intended to increase the thirst of the customers, as well as to raise the density of the porter. Acidity of an acetic kind was also detected, to a great extent. As far, therefore, as the Lancet Commission is concerned, the London publicans and brewers come out with tolerably clean hands; and, with regard to bitter beer, have absolutely gained a character which they had not before."
"A manual of domestic economy: suited to families spending from £100 to £1000 a year" by John Henry Walsh, 1856, page 324.
This text implies that large-scale adulteration of beer with poison was indeed a thing of the past by the 1850's. Though note how Porter, as ever, was the object of most suspicion. All that investigations could determine for certain was that beer sometimes had water or salt added to it. The oldest tricks in the Publicans Book of Tricks. Watering - or at least returning the slops to the barrel - still goes on, I'm sure. It's just too easy.