It is recommended you do not drink the tap water in Tanzania. Bottled water (both before and after the trek) is at your own expense, however, during the Kilimanjaro trek you will be provided with drinking water. This water is collected from fresh mountain streams and then purified with tablets by your guides who will replenish your empty bottles/thermos flasks on a daily basis. For peace of mind, you are welcome to add your own chlorine or iodine tablets instead, if you are uncomfortable drinking the water provided.
