cimitidine wont extend your rush. the only way to extend your rush is to increase the amount of drug consumed. the duration is directly correlated with concentration, the rush of shooting up comes from the extremely fast conversion of your drug of choice into dopamine and endorphins. more dope = more dopamine = better, faster, stronger rush.
theoretically to manipulate the rush's duration and intensity without simply taking more of a drug, you have to hasten the rate-determining step in which the drug is converted into 'feel good' neurotransmitters(or add a catalyst/enzyme to decrease activation energy). the slowest step of the reaction determines how fast the whole series of mechanisms or overall reactions take place.. usually, the faster the drug passess through the blood-brain barrier, the more intense the rush is. for example, heroin and cocaine both have amazingly awesome abilities to pass through the BBB with such ease which allows for the most extreme and pleasurable rush. basically to my understanding, for a drug to have a "rush", the reaction that takes place during passage through the BBB into the CNS MUST be faster than the slowest step that occurs throughout the whole series of reactions. this would only make sense because this postulation correlates well with the idea that to increase the concentration of heroin/cocaine, you increase the intensity/duration of effects.
cimitidine, and im talking out of my ass here, mainly affects the peripheral systems such as liver enzymes and NOT the Central Nervous System (CNS). the rush that takes place when a user injects a drug happens solely in the CNS, the rapid and almost instantaneous conversion of the drug happens in your brain which is the CNS. this is where monoamine oxidase breaks down heroin molecules into mainly morphine molecules which then bind to opiate receptors and thus the rush takes place. afterwards, the drug passess through the liver, which would where tagamet/cimitidine would come in. note this is AFTER the rush is good and done with, but the drug is still felt.
lets say you inject heroin+cimitidine. essentially, the heroin would pass the BBB much quicker than the cimitidine (if cimitidine EVEN CROSSES THE BBB AT ALL) and the reaction (rush) would occur. after that blissful rush (who knows how long) then the cimitidine would FINALLY kick in and prevent the degradation of the morphine molecules.
i think it would be impossible to add a catalyst to the CNS, and i think it would be impossible to shorten the time it takes for a drug to pass through the BBB by taking another psychoactive substance/.