Descaling Nespresso with citric acid Nespresso instructs its customers only to use lactic descaling solution or the guarantee may be void.
However lactic acid can be hard to get by and Nespressos own solution is heart-stoppingly expensive.

What could the reasons be not to use citric acid and is this a valid argument for the Nespresso machines?
My opinion is that you can use citric acid. That's all that I use and I use it with my espresso machines, drip coffer maker, and I have used with Nespresso and Keurig machines. It is food safe. I buy it at my local beer and wine supply store. I have read to to use 12 - 18 grams of citric acid to a liter of water. I believe many descale solutions are made from citric acid. I use 3 1/2 teaspoons of citric acid to 40 oz of water to make my descaling solution. You can follow the manufacturers suggested routine of descaling.