I’ve read a lot of stories and was cruising around on theCrimeLibrary.Com and a lot of the women were using rat poison as a means to eliminate their husbands and other forms of “human pests” 
These seem to have happed before the 1950s. I read that rat/mouse poison works by stopping the blood from clotting. But it seems this is not the same as it was when these women were poisoning their husbands.
First of all did they reformulate rat poison? If so what used to be in it, say before the 1950s?
arsinic
In the 60’s the mole, mouse and rat bait of choice was Poison Peanuts with powdered arsenic. One brand was Sweeney’s Poison Peanuts which don’t have a peanut covered in arsenic today. They always ate the bait and died unlike the current pellets that they often let just lie in the hole. One thing my parents always drilled into my head was to never eat peanuts that I found somewhere ever. This wasn’t just for their house. The peanuts were covered in a pink or green powder.
Thallium.
Odorless and colorless it was the perfect domestic poison.
Link:
Even though rat poison containing thallium was banned in 1972, accidental poisonings from old rat poison still occur, especially in children.
A more modern rat poison is Warfarin or Prolin a anticoagulant. Dairyland Rat Poison was very deadly, but not around until the mid 1900’s. It was developed in the University of Wisconsin, because of a heard of cattle that were bleeding to death from eating some spoiled clover hay. Rat Poison Distribution by Boy Scouts
I thought they also used Strychnine, but I can’t find a source.
In smaller doses, Warfarin (Coumadin) is prescibed to cardiac patints to prevent blood clots.
The article I linked explains how it was developed into a usable drug for people.
I read that Warfarin isn’t used so much any more as mice and rats have gotten used to it so it isn’t as effective as it once was.
I didn’t realize arsenic was used, but I guess it was. I know you always read in the old days women would use arsenic for some cosmetic reason and to get the arsenic they would soak flypaper and get the arsenic from that.
I always see a lot of TV and movies (which I know are not always true to life) and it seems the villian puts the rat poison in a drink or food and then the victim eats it and falls over sick in a matter of a minute or so. But I know with the modern rat poison that 'causes the blood not to clot (something to do with interfering with vitamin K) this can take a few days to take effect
The active ingredient in things like D-Con is now brodifacoum, which is also an anticoagulant that works much like warfarin. It also has a much longer elimination half life, so that the rodent can die from repeated sublethal doses, which wouldn’t build up if warfarin was still used. This persistance causes some groups to claim that it should be banned on the basis of secondary bird poisonings.
anticoagulants are still currently used in consumer poison baits. Warfarin needs higher doses and multiple feedings, newer types can work with a single feeding. Advantage to anticoagulant is the fluid loss will prompt the rodent to seek water which often leads it to go outside the home.
Wikipedia reveals some fascinating strategies for killing rats: metal phosphides generate phosphine gas in the digestive system; zinc phosphide was the standard rat poison before anticoagulants became popular, as strychnine became scarce during WWII. Forms of vitamin D cause hypercalcemia in rats (and dogs and cats); narcotics, phosphorous, cyanide, and nerve poisons have all been used; and antibiotics are used to prevent intestinal flora from producing vitamin K, which might counteract some anticoagulants.
Yeah, I’ve been taking it for a few years. Got to get a blood test every month to be sure it remains in the tolerable dose range. It still results in rather profuse bleeding when I cut myself.
I believe for rats and mice, it causes severe internal bleeding that offs them.
I am very, very careful to not overdose. I keep the bottle hidden from my wife, just in case. 
My understanding is since mice and rats routinely crawl into little spaces they bump themselves. Normally this bruising will bleed a tiny bit but with the newer rat poisons they bleed to death internally.
But in the Crime Library, there was an awful lot of “black widows” that seem to have gotten rid of their husbands with rat poison and I was like "There must’ve been something different in it way back then. Because the effect seemed to be more immediate.
it would take a boatload of anticoagulant poison bait to kill a person. i think the poison used in the 50s and earlier would have been the arsenic poisons which was a hardware store item.
That isn’t true. All animals develop minor haemorrhages all the time, it’s just due to wear and tear. Normally they clot fast and are never noticed. Add anticoagulant and they become fatal. One of the main sites of bleeding with anticoagulant poisons is into the joints. No external injury is required.
Added to that, rats nd mice are simply too light and too weak to cause bruising by bumping into things
As others have noted, rat poisons commonly include thallium, arsenic, metal phosphides, strychnine, cyanides and fluroacetate. Any of those will kills you stone dead in short order.
So why did they switch? Did rats become used to the poison or is the anticlotting thing kill more?
Aside from brodifacoum and related Vitamin K1 inhibitors, the other common ingredient in newer rat poisons is bromethalin which causes cerebral edema. There ate also still some around that contain cholicalciferol which is nasty, it causes hypercalcemia and it very difficult to treat. Before there was a Animal Poison Control to call we made index cards for common poisons that we could refer back to, the one for Rampage (the cholicalciferol variety) “dead dog”. I realized that wasn’t helpful and added other information later, fortunately we never saw much of these but I did see one case several months ago and we got the dog soon enough to be able to treat it. So it must still be on the market.
the anticoagulant poisons are a much less hazard to humans and larger animals. accidental, and to your question intended, poisoning was much easier with the other poisons.