The old sachets of Screech powder (or crystals) were provided in your 24 Hour Ration Packs to make a refreshing fruit-flavoured drink for the hot days on exercise. There were several flaws in the scheme.
1. The only receptacle you had to mix the drink in was the issue black water bottle. The sachet had too much powder to make a single bottle, so actually getting a mix that was palatable was very difficult. What was left in the sachet was either lost, thrown away or leaked out to make your pocket the most attractive point for flying and crawling insects for miles around.
2. Once mixed, Screech was only drinkable when it was fresh out of the fridge. It's not normal to have a fridge handy when you are feeding off 24 Hour Ration Packs so it had to be drunk lukewarm at best - and on a hot day stored in a black plastic bottle, hot.
Screech was still a great favourite with the troops, and the only reason that could be possible was, I believe, because it was addictive. At first men would turn down the sharp, strong-tasting drink. It was difficult to mix and didn't taste good enough for all the effort. They'd soon be adding a ¼ of a sachet to give their water a little bit of taste and throwing the rest away.
As time went on the ¼ mix was no longer enough and the mix would get stronger and stronger; Screech would get scarce and new ration boxes would be raided and the sachets of powder would be removed. Soon the crystals would be poured straight into the mouth and camouflaged troops covered in cam cream would have bright orange, yellow or green mouths. Anyone with a normal coloured mouth would have their webbing raided and any sachets of Screech would be confiscated and passed amongst the rest.
In very bad cases, just before endex, the crystals would be going straight up the nose, thereby confirming the user as a hardcore Screech-whore. The only reason for such behaviour is the addictiveness of the pretty coloured crystals. Screech can also be sold around camps and what-not. It's really funny to watch cadets and young students alike go hyper.