WARNING
Use jet (-1 charisma) or mentats (+2 charisma) may do lose your merc : if you travel, your party points decrease because use jet or mentat wear off, there is a possibility to lose your merc in encounters.
That is most likely completely unrelated to pp, though.
There are presumably several bugs revolving around followers that would lead to loosing them in (or more precisely right after) encounters, suspected to be related mainly to their combat timers.
Piling up pp with drugs to recruit followers beyond your max pp should not present a problem. That's from general experience with mentats/beer, I cannot speak for jet. In fact, boosting charisma with drugs before recruiting mercs is recommended to begin with.