Ive done the 2 weeks on 2 weeks off before several years ago and I didnt do it for long.
Recently I decided to do clen and t3 to get me down to single digit bf% and I did some research and several clen advice articles talked about adding ketotifen while on clen so you dont have to take 2 weeks off.
The theory being that it prevents or at least significantly reduces the down regulation of the beta receptors that happens quickly with clen (necessitating the 2 week break).
It seemed to make sense and well written explanations so I am experimenting with a six week straight cycle with ketotifen.
I am only on day 8 so I cant give results of my experiment, in a couple of weeks though, if my hands are still shaking, Ill have my answer as to whether keto has allowed the clen to keep working.
Others may have more experience with adding ketotifen, its an antihistamine taken just before bedtime as it can make you sleepy. Or you can just google clen and keto and read all the stuff I did.
I also am taking t3, yohimbine with the clen and keto for a six week cutting cycle (if I need to take it that long, 8 days in and seen clear accelaration of fat loss).
Dont know the answer as to why not take it on pct, many others have done it to help keep muscle, dont know if it works or if this is some glaring reason not to, so far in my research, I cant recall anyone saying not to.
I am taking 500 mg of test/week while on the t3 to negate the muscle catabolism that occurs with the fat catabolism of t3.
I wouldnt worry about the expiration date, thats just a date where the manufacturer can no long guarantee it to a certain potency. most drugs are still useful past the expiration date for a very long time, its not dangerous, just may not be as potent, but its not a magical date where all of a sudden its bunk
Hope that helps, I am sure others can chime in with more advice, but definitely look into ketotifen and clen on google. Theres a buttload of articles and posts on the combo