It was taught in a Baraisa: water that was left uncovered should not be spilled out in a public area, nor should one knead clay with it, nor should one lay in the dust with it, nor should one give it to his animal, nor the animal of his friend, to drink. (Bava Kamma 115b)
Don't Drink That Water!
The rabbis of the Talmud were very worried indeed about the health effects of water that had been left uncovered. This concern was codified by Maimonides, and later by Ya'akov ben Asher (d. 1340) in his famous halakhic work called the Arba'ah Turim.
טור יורה דעה הלכות מאכלי עובדי כוכבים סימן קטז
דברים האסורים משום סכנה
יש דברים שאסרום חכמים משום סכנה כגון משקין שנתגלו שיש לחוש שמא שתה מהן נחש והטיל בהן ארס אפי' אם שתו מהן אחרים ולא הוזקו אין לשתות מהן שיש נחש שהארס צף למעלה ויש שארס שלו מפעפע עד אמצעית המשקה ויש שהארס שלו שוקע לשולי הכלי לפיכך אפי' שתו ממנו אחרים ולא הוזקו אין לשתות מהן דשמא ארס של הנחש ששתה מהן שוקע ואלו המשקין שיש בהן משום גילוי מים יין חלב ודבש ושום כתוש
Tur, Yoreh De'ah 116. Things that are Prohibited Because they are Dangerous
There are things that the rabbis of the Talmud prohibited because they are dangerous. For example, liquids that were left uncovered, because of the possibility that a snake drank from the water and expelled some of its poison into them. Even if others had drunk from the liquid, and not been injured, one should not drink from them. For some snake venom floats on the surface, and some sinks to the middle and some moves to the edges of the vessel. Therefore, even if others had drunk and had suffered no harm, one should not drink from them, for perhaps the venom from the snake that had drunk the water had sunk to the bottom. The following liquids should not be drunk if they were left overnight in an uncovered vessel: water, wine, milk, honey, and crushed garlic...
The normative Code of Jewish Law, the שולחן ערוך agreed, but added an important caveat:
שולחן ערוך יורה דעה הלכות מאכלי עובדי כוכבים סימן קטז סעיף א
משקים שנתגלו, אסרום חכמים דחיישינן שמא שתה נחש מהם והטיל בהם ארס. ועכשיו שאין נחשים מצויים בינינו, מותר
The rabbis forbade drinking from liquids that were left uncovered,. They were concerned that a snake may have drunk from them and expelled some of its poison into them. But now that snakes are not commonly encountered, this is permitted. (Shulchan Aruch Yoreh De'ah 116:1)
So today it is permitted for us to drink from an uncovered pot, but only in a place that does not have a problem with poisonous snakes. Which is not helpful. There are poisonous snakes in nearly every state in the US, resulting in about 2,000 human envenomations each year, and we have noted before that Israel has its own problem with snakes, including the Palestinian Viper. The World Health Organization estimates that snakes kill between 20,000 and 94,000 people per year. So exactly where this leniency of the Shulchan Aruch might apply is not clear.
But is drinking snake venom indeed dangerous? Maybe not. In 2012 India Today reported that police in New Delhi had seized about half a liter of snake venom to be used "in high-end raves planned for Valentine's Day in and around the national capital." Apparently the venom, when ingested, produces a euphoric state. Who knew?
Video evidence - Drinking Cobra Venom
It is really hard to find any peer-reviewed scientific studies about people drinking snake venom, because, um, it's a silly thing to do. But that doesn't mean it hasn't been done. So where could we turn to find people doing silly things? YouTube of course. This video involves drinking the venom directly from spitting snake. Apparently, these kind of human interest stories are popular in India.