Zamzam Water is the best, freshest, most beneficial, most beloved and most precious water there is. Further, Zamzam water was dug up by Jibril and the source that Prophet Isma'il(AS) drank from.
In the Sahihain it is narrated that the Prophet(SAWS) said to Abu Tharr, who remained at the Ka'bah for 40 days without food (or drink) living on Zamzam water: "It is a nutritious food." Muslim added that the Prophet(SAWS) said: "And a cure from ailment."
Furthermore, Ibn Majah narrated that Jabir bin 'Abdullah related from the Prophet(SAWS) that he said: "Zamzam water is for whatever was intended behind drinking it."
We were told that 'Abdullah bin Al-Mubarak once said. "O Allah Ibn Al-Mu'ammil told us that Muhammad bin Al-Munkar related from Jabir(RA) that your Prophet(SAWS) said: "The water of Zamzam is for the purpose it is drank for, and I drink it to relieve my thirst on the Day of Ressurection." This Hadith is of the Hasan (Sound) type.
Many other people and I have tried drinking Zamzam water for the purpose of being cured from various illnesses, and by the will of Allah, I was indeed cured from several illnesses. I also saw some people who lived on Zamzam water for a long time, fifteen days or a little longer without complaining from hunger. Such people would perform Tawaf with other people at a normal pace. Someone even told me that Zamzam water was his only food and drink for forty days and that he still had enough strength to fast and perform Tawaf many times.
Taken From: Healing with the Medicine of The Prophet(SAWS) by Imam Ibn Qayyim Al-Jauziyah