FAQ

Beth has some particular beliefs. It's no unusual for people that open-minded to take recreative drugs. At the start of the movie, she offers to her guests the content of a vial that has, among other ingredients, Ketamine. As Wikipedia states, it "is a medication used mainly for starting and maintaining anesthesia. Other uses include sedation in intensive care, as a pain killer, as treatment of bronchospasm, as a treatment for complex regional pain syndrome and as an antidepressant. It induces a trance-like state while providing pain relief, sedation, and memory loss. Heart function, breathing, and airway reflexes generally remain functional. Common side effects include psychological reactions as the medication wears off. These reactions may include agitation, confusion, or psychosis... Ketamine's use as a recreational drug has been implicated in a number of annual deaths... Ketamine may increase the effects of other sedatives, including, but not limited to: alcohols, benzodiazepines, opioids, and barbiturates." (https://en.wikipedia.org/wiki/Ketamine, emphasis added).

As the film unfolds, we only know for sure that in the Prime Universe, Lee took it in order to take some sleep and Beth took it with a glass of wine in order to battle with the stress the whole situation has lead her to. Although the exact amount of ketamine inside the vial is never explicitly given, as Beth says it's only a little, they both show symptoms of its consumption. However, it's possible that contrary to what she repeats several times, she could have put some of it in the food served in the dinner. While this does not explain the entire movie's confusing story, it could explain several facts, such as the way Mike falls taking more and more wine and the way Laurie flirts to Kevin caring so little of being caught. An artistic liberty could have been taken with this drug as justification to Beth's nasal bleeding near the end. Even possible hallucinations and confussion are prevalent in the oneiric atmosphere of the film, therefore, is open to interpretation if they all took the drug or not.

"Schrödinger's cat is a thought experiment, sometimes described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat that may be simultaneously both alive and dead, a state known as a quantum superposition, as a result of being linked to a random subatomic event that may or may not occur. The thought experiment is also often featured in theoretical discussions of the interpretations of quantum mechanics. Schrödinger coined the term Verschränkung (entanglement) in the course of developing the thought experiment." (https://en.wikipedia.org/wiki/Schr%C3%B6dinger's_cat)

Inside the context of the film, Hugh's brother takes cue from this thought experiment to explain, more or less, the many worlds theory, which states there's an infinite versions of the universe conforming the multiverse. If we think of the characters as the cat inside the box, the Coherence what happens when the cat gets out of the box alive and ends up observing and interviening inside several parallel boxes and finding copies of itself living in some of them, and dead in others, all happening at the same time.