ok first: stimulants are not all the same. they each have their own unique effects. Since I'm on dextroamphetamine at the moment, I'm going to give you more information than you really need.
second of all, an "excuse" to take stimulants is different from a reason to take stimulants:
and excuse to take stimulants means that you really wanna get high off of say, adderall or dexedrine, and you justify your need by taking the drugs "to study" (even though you never study), or to increase your performance in video games, or simply because you "feel tired all the time", and by convincing yourself that your use is justified because of how you used it, you are in denial. If you try to make up an "excuse" for your drug use, then you shouldn't be using stimulants. (I'm not directing this at OP, but rather speaking hypothetically)
a reason to take stimulants is because you have severe ADHD (like myself), because you need to stay awake for a job that requires you to stay awake for abnormal periods of time (truck drivers, 24-hour restaurant/market/convenience store clerks), and (people may disagree that this is a good reason, but I believe it is) creating music or any other form of art, from writing novels to graphic design. Also, taking stimulants recreationally is a good reason to take it IMO. Although due to the many different specific substances and ROAs, not all stimulants have a good reason to be used: Methamphetamine (all ROAs aside from oral and rectal) cannot be used responsibly IMO. MDPV shouldn't be taken by anyone for any reason. MDMA can be used entheogenically, but when IV'd or insufflated, it becomes more similar to methamphetamine, and therefore does not have entheogenic properties.
With cocaine, however, there is some controversy: The 80's was known as a time where a lot of people were very rich, and a lot of people were snorting very pure cocaine, and becoming very successful businessmen or lawyers or politicians while doing so. Cocaine is intensely euphoric, yet works better than many other stimulants to treat ADHD IME (intranasal only). This nationwide corporate cocaine binge ended when freebase and IV cocaine started becoming more and more common, and the functional coke-heads became burnt-out crackheads. I wouldn't recommend using cocaine for any reason other than recreational, especially with the quality of coke nowadays (even when I get "really good" coke now, it's never as good as what I used to get in 2010-2011, not even remotely close). The price and risk of death is too high and the purity and duration is too low with cocaine, so if you will use a stimulant to be "productive" at your job, or at school, or for a personal project, than let me offer you a list of suitable stimulants for your purposes, as well as some suitable productive situations in which using stimulants is appropriate:
Stimulants in Order of Preference:
(All drugs listed must be taken PO)
1. Dextroamphetamine/Adderall ER/XR formulations
•Dextroamphetamine is much less jittery and a lot less physically uncomfortable, which is a benefit for most (like myself). It's the only significantly CNS active part of racemic amphetamine, so the "high", which lasts around 2-4 hours (depending on tolerance) is all due to d-amp. It has a half life of 6 hours, and generally doesn't disturb sleep patterns unless taken late at night.
•Many users claim that the 25% levoamphetamine in adderall (or 50% in the generic amphetamine sulphate tablets/capsules) helps to "kick their ass" into studying harder. However, levoamphetamine has a long half-life, and will keep you awake for up to (and often exceeding) 12-14 hours after dosing. Levoamphetamine is PNS active, and will make you "tweaky" but not euphoric. I find that levoamphetamine is actually helpful, but the d/l ratio should be 6.1 rather than 3.1.
•Amphetamines are neurotoxic and very addictive.
2. Lisdexamfetamine
•Pro-drug for dextroamphetamine. Slowly metabolizes into d-amp in your liver, acting like a very smooth, slow-release dexedrine.
•It's not very stimulating in therapeutic doses, and his very little, if any "crash" to speak of.
•It does not interfere with eating and sleeping schedules IME and is IMO exclusively suitable to studying, rather than to work late shifts or other "productive" activities that stimulants could be used for.
•Is the most "relaxing" stimulant on the market IME, and I've accidentally fallen asleep 3 hours after taking 120mg of it (there were no benzos or any other downers/opiates combined with it).
3. (Dex)methylphenidate XR
•Structurally similar to an amphetamine, yet it is not considered an amphetamine, and has a completely different mechanism of action on the brain (amphetamines are dopamine/norepinepherine agonists while methylphenidate is a DNRI [dopamine-norepinepherine reuptake inhibitor])
•Similar to cocaine in mechanism of action (they are both DNRI's, although cocaine is a SDNRI), duration, effects, dangers, and dosage (provided that the cocaine is pure, which it will not be).
•Will be ineffective to stimulant-experienced individuals. If abused, the medication will stop working. Insuflating the crushed tablets or plugging/IVing are common practice, but those ROAs will not help you be productive.
Activities where stimulants are appropriate:
•Studying
•Extreme fatigue
•Long work-shifts
And that was the definitive guide to being productive on stims!