Like "BrokeDownPalace" said; if your high on heroin and trying to hide that fact, pin-point pupils would be one of the last things i would worry about covering up. Perhaps these people (parents, significant other, roommate(s)) who suspect you are high when in fact you actually are, are suspicious because of other tell-tale signs of heroin use.
For example:
1) dark circles &/or bags under the eyes,
2) a glazed over--glassy eyed appearance,
3) droopy eyelids,
4) eyes uncontrollably rolling back into your head or become "cross-eyed"
5) slurred speech,
6) "nodding" in and out during conversations, at the diner table, on the couch etc... Basically, unless you're on drugs, especially on heroin and/or other opiates, you shouldn't be nodding in and out of consciousness anywhere nor anytime, except when you are in your bed, just woke up, or are just about to go to sleep
7) frequent and uncontrollable itching (especially itching of the nose if you snort your heroin),
??? = if i forgot any other major signs that even the drug naive are aware of and look for, please add to the list....
OK, HOW TO FIX THESE ISSUES: (the remedies will correspond to the number assigned to each physical clue listed above)
#1 = Use some sort of make-up that is specifically made to cover up bags under eyes. Sure this make-up is made for women but men can use it too, or if you feel more comfortable or don't usually wear make-up, then saying one is tired should be sufficiently explanatory... should be....
#2 = eye drops for allergies, such as naphxxx or opcoxxx
#3, #4, #5, #6 = Caffeine, caffeine, and more caffeine! Any type of energy drink should do the trick for about 30-60 minutes after drinking it, depending on how high you are and your caffeine tolerance. Also, sugary foods work, like candy etc...
#7 = an antihistamine like Benadryl or anything w/ diphenhydramine (sic?) in SMALL doses will get rid or at least help with the itching. WARNING: antihistamines often make people drowsy, sleepy, tired, etc..., in fact, people will take these OTC drugs w/ heroin to add to the "nod" heroin gives, essentially a dirty way of making the heroin feel more potent. Thus, while it may make you stop itching your skin off, it may make you look more drowsy, so its kind of a wash. If you know that these drugs don't really make you tired then go for it, but take half of the normal recommended dose on the box and then titrate upwards as needed. IMO, looking sleepy and nodding out is a way bigger red flag than itching is...
-Back to the OP's original question, Belladonna has been used for centuries to dilate pupils. Now, Belladonna pills are sold in most large pharmacies and in natural food stores as a type of OTC alternative medicine. i've only seen it sold, never actually looked at what its supposed to cure, but regardless of its intended use, it should dilate pupils enough but not too much when taken at the recommended dosage.
DISCLAIMER: I am in NO WAY advocating people use heroin, or anything i mentioned to mask their use, but if putting on some make-up and drinking a large cup of coffee keeps you out of jail, a roof over your head, and prevents you from falling asleep at the wheel or worse, then its worth it.
be safe ya'll