The way I understood it is, that if you're at full points, you get a boost. If you're not at full points, you get some psi points back but you can't go over your natural maximum. I can give examples.
That is my understanding. "These drugs restore PSI if taken when the Traveller has already spent PSI points or temporarily increase the Traveller's PSI if taken when they are at full strength." You have to be at full natural strength to get a boost, so if you're down 3 points and take a dose of Standard you are at your normal PSI, not your normal PSI+3.
If you are at your full PSI and you take a dose of Standard you're at PSI+4.
PSI the attribute and psionic points are not two separate things. "Using a psionic power costs a number of PSI points, temporarily reducing the Traveller's total. As the characteristic DM for all psionic skill checks is determined by the Travellers' current PSI total, it gets harder and harder to use powers as the Traveller's PSI declines."
So if you use a psi-drug to boost from your natural PSI 10 to PSI 14 you now have 14 PSI points to burn on powers, and a characteristic DM of +2 (where it was +1 at your normal strength) to all your PSI rolls. Until you start spending that PSI and your current total goes down, that is.
Note that you use your PSI rating before you spend any points to determine the characteristic modifier to your roll, and if you fail your roll you only use 1 PSI.
Detailed Example of Multiple Psi-power and multiple doses of psi-drug use:
I've boosted my original PSI 10 to PSI 14 by taking a dose of Standard. Now I want to teleport myself with my clothing a Very Distant range - around 400 kilometers - to a different city on the planet I'm on to see why we've lost contact with it. Teleportation normally costs 2 points. It costs another 2 for teleporting with clothing and up to 10kg of gear. The Reach of Teleportation is normally Distant, but can be increased to Very Distant by spending double the points to increase the range by one range band. The full cost for the teleport will therefore be 8 PSI. Teleportation on a planetary surface is restricted to Very Distant at most, and I have to have a good mental image of the location I'm teleporting to - I'm teleporting into a city park I've visited before in the middle of the night, so no one is likely to see me.
My Teleport check will be Difficult (10+) instead of Average (8+) because I am taking my clothing and limited gear with me. There is no increase in difficulty for the further distance, just PSI cost. My current PSI 14 gives me a +2 DM, and let's say my Teleportation skill is also 2, giving me a +4. I have to roll a 6+ to succeed, better than even odds. If I had tried to teleport without having taken the Standard first I would have to roll 7+ to succeed, and I would have only 2 PSI when I got there.
I rolled an 8, so the Effect of my roll was 2. I teleport, and my PSI drops by 8 points to 6. I am also disoriented for 2D-2 x10 seconds, because I teleported to a destination not within my line of sight. I roll a 8 on this and am disoriented for 60 seconds.
After the minute I am together enough to have a look around. I realize that the entire city around me is on fire, and I decide I want to teleport back to my origin immediately before the flames reach the park. Because my PSI is now 6 I have a +0 Characteristic modifier, so to return to my destination with my clothing and gear will be a roll of 8+. Very Difficult (10+), with PSI 0 and Teleport-2. It will also cost me 8 PSI, and I only have 6 PSI, but you can use more PSI than you actually have in a single power use, as long as you have
some PSI (more than 0).
I roll a 7. Fail. My PSI is reduced to 5 by the failed attempt and I don't go anywhere. Dropping to PSI 5 means that I now have a -1 DM on PSI rolls. To teleport back now will need a roll of 9+ (and, again, more PSI than I actually have, but I can still make the attempt and take the extra 3 PSI as damage points to my END if I succeed).
Since the situation is desperate, I decide to use another dose of Standard so I can make the second attempt at an easier difficulty.
But, because I know I'm going to be making an END roll for taking a second dose of Standard within a one week time period, I decide to first use another of my PSI powers - Enhanced Endurance. My END is normally 8, for a 0 modifier. I decide to boost my END to 12 to give me a +2 DM. This takes 4 PSI and an Average (8+) Awareness check. I have Awareness 1 and a current PSI of 5 (-1DM) so the roll has to be 8+ to succeed. I roll an 8, and my END immediately raises to 12 and my PSI drops to 1. My enhanced END will last for the Effect of the check x10 minutes. Since I rolled exactly what I needed (Effect 0) the Referee rules that it will last 1D minutes before it starts to drop towards my normal END. I roll a 1, for 1 minute of enhanced END.
I now take the dose of Standard. The END check to resist damage is Average (8+), with a -1DM for having taken two doses within one week. With my +2 END modifier I need to roll a 7+ to avoid damage. I roll an 8, and avoid taking damage from the dose. Aside from losing permanent PSI, I would have also taken 3D damage, which could knock me unconscious on a high enough roll, leaving me to be burned alive.
I'm well below my normal maximum of PSI 10, so I recover points instead of receiving a boost. I go back to PSI 7. I now have a PSI characteristic modifier of +0 rather than -1. I technically don't have as much PSI as I will need to teleport back (8 points), but as long as I have above 0 PSI I can make the attempt. The difficulty is again Difficult (10+) with no modifier from PSI and a +2 DM for my Teleport skill, meaning I have to roll an 8+. I roll a 9, and am teleported back with an Effect of 1. My PSI drops to 0 and I take 1 point of damage, dropping my END to 11.
I am disoriented for another 2D-1x10 seconds, but I rolled a 4 on the dice, so it's only 30 seconds before I can tell the rest of the party that the city is on fire.
My END drops to 10 a minute after I used the Enhanced Endurance power, dropping by an additional point each minute until I'm back to 8 in another two minutes (four minutes total from when I used the power). The point of damage I took from spending more than my PSI in teleporting back was absorbed by my enhanced endurance, causing it to wear off a minute early but otherwise leaving me unharmed.
My PSI stays at 0 until three hours after I used Teleport, when it goes up to 1. It takes me 9 hours after that to recover my normal PSI, a total of 12 hours after the second teleport.