There are some drugs I keep, like metronidazole, for the occasional runny poop situation. But I know the exact dose each dog takes, and I always call my vet (who knows I do this) and run it by her. She says "yeah, proceed," and she charts it.
If my dog were to have a bad reaction, I could bring him in and it wouldn't be a surprise to anyone.
I once had bronchitis, and my doctor suggested using an inhaler. I told her that my husband, (who has asthma) had a 3 month supply of them (we use a mail order pharmacy because it's cheaper, and he very rarely uses a rescue inhaler.). She told me that she would write the prescription for that particular brand, and if I chose to "fill" my prescription at my medicine cabinet rather than at the local drug store, she had no qualms with that. But she did make sure that the prescription matched what I wanted to use because the computer automatically cross-checks contraindications with the meds I already take and with my allergies and such.
There are too many risks inherent in self medicating. Even with OTC drugs, it's not a good idea to self-medicate for anything more than the simplest of issues. For example, approx 60% of "yeast" infections that women self-treat (often repeatedly) are actually bacterial infections, STD, other infections, etc. So these poor women are spending quite a bit of money and are miserable for days or weeks or longer using products that will NEVER cure what's wrong with them. Sure, the folks at Monistat that got their product approved to go over the counter not too many years ago love this. But it's not good for women's health (or their sanity).
So you could end up dropping a lot of money then still needing to go in to the doctor's office anyhow in that case.
That's just an example.
In the case of using someone else's drugs, contraindications can and do occur. Google is not a substitute for a medical degree. It's just not. There is a lot of info available on the web, but there is a lot of bad information on the web too. So you can't trust that.
I've sat around the office or other places and listened to people talk about what they're "sure" they have wrong with them. I just shake my head. I'm not a doctor, but I'm pretty conversant on medicine, enough to know crazy diagnoses when I hear them. There are some things I can pretty easily self-diagnose, and most things, I make an appointment for. I can be 98% certain, but I still go in.
If something goes wrong when you self medicate, you need to go to the doctor. You want the doctor to know exactly what you took, to know it was a fresh prescription, and what it could have interacted with. If you're pulling stuff off the shelf, you can't say for sure. Some meds lose their efficacy in light or heat, for starters. Dosage may change as you age, or if you have additional health issues, like your blood pressure has gone from normal to elevated.
It's just not worth it. For me, it's not even a moral issue. It's just a practical matter.
So, as a practical matter, do I save some prescriptions? Yes, cough syrup with codeine, I save for the next cold. But at the end of cold and flu season, I pitch it. Metronidazole for the dogs, I save (but I store it properly and ditch it long before the expiration date). For dogs that have recurring issues (or a drug that may be vital in an emergency), I may save something else, if the vet and I think it's a reasonable thing to do. Or
Of all drugs, antibiotics should never be used as a partial prescription. They are prescribed as a treatment course and should be used that way. So if you don't finish them (if you need a different abx), toss what you have and finish the new ones. Don't use the partial Rx another time. Otherwise, you really risk creating resistance, which can be a big problem down the road.