The Fremen knew of the effects of the Water of Life and used it to make their Reverend MOthers. They were taught this by a Reverend MOther in extremis following the Missionaria scheme (JEssica guesses). BEfore that other drugs were always used. However once they discovered how effective the Water of Life was no other drugs could match. Also, if a Reverend MOther was made one via Water of Life she couldn't be influenced by the other drugs. During Leto's reign the Reverend MOthers sometimes used the other drugs because they had a short supply of melange but it was comparatively ineffective. BAsicly the spice doesn't make a Reverend MOther but changing poison does. It has to be a special poison that taxes the Reverend MOther. While she is focused inward on changing the poison her other abilities are made available. Jessica does comment on the Water of Life saying something to the effct of: They have discovered how to use pure melange to make Reverend MOthers. I like everyone else assumed that they had always used Melange but upon further investigation I realized the truth. Even Brian HErbert managed not to screw up there and in his books Melange is not used to make Reverend MOthers among the BEne GEsserit. The first illuminating poison was found in the Battle of Corrin on Rossack by Anirul (or whatever her name was). Leto II in GEoD mentions that discovery on Rossack.
