The ancients were more prone to using muscle relaxants, mainly because there are a number of herbs that can be used in this manner. Hemlock, the herb that was used to kill Socrates, is among the more prominent examples of herbal muscle relaxants that could be used as poison. Curare, a South American herbal substance used in poison arrows, has recently gained renown for its potentially lethal muscle-relaxing properties. Even now, these herbal poisons see much use, though they have faded from the public eye in favor of more readily-available toxins that don't require so much preparation or knowledge of herbal lore to use properly.
Arsenic, cyanide, and strychnine are among the more commonly available ones. Arsenic, in particular, has seen much use as both a preservative agent and a poison over the centuries. While there are no official records of such things happening, most historians today believe a number of sudden deaths of various historical figures might have been due to arsenic poisoning. Cyanide is a more recent poison, though it rapidly gained a reputation for being a very effective toxin that was also simple to use, usually lacking the same learning curve involved with the use of arsenic. Strychnine, while almost as potent a poison as cyanide, is not as readily available and is easier to detect on standard toxicology tests.
Doctors and nurses have access to a wider range of possible poisons than the average person, with the same holding true to a degree for pharmacists. Medical professionals are aware that certain compounds or substances, while normally safe for human consumption, can be lethal in the right situation, or mixed with the right substances. For example, high levels of potassium can be lethal for those with kidney or liver problem. Among muscle relaxants, succinylcholine can be a particularly good choice because it does not automatically show up on a standard toxicology test. Since the effects of the drug can look like respiratory and heart failure, even to an experienced medical professional, it makes for an effective poison.
Antifreeze is also a commonly used poison, one which most people would never be able to detect outside of a toxicology test. Antifreeze is a compound that is highly similar to sugar, which gives it a sweet taste that makes it easy to hide in nearly any sweet-tasting drink. Popular choices include iced tea, fruit juices such as orange or grape, and occasionally, fruit-flavored smoothies. However, antifreeze leaves behind a few trace elements due to how it metabolizes, which means that it can be easily detected if the coroner or the police suspects it was used.
