My father always did that and still does so. Here is the problem though. Mistakes can be deadly. He is not a Doctor. He is not keeping up with the changes and a decision of the past when continued today can hurt him. There are too many other issue where unless you are a dedicated expert you can really hurt yourself if you are one of those who self medicate simply because it seem possible. Majority of the world does that, it seems, to one extent or another.
For an American it is hard to believe such a practice of self medication on prescription drugs can be prevalent. However, similar practices do exist in other fields, with effects less disastrous than human health but no less damaging to the institution they serve. Why do senior managers casually ignore the market research managers? Why do strategy decisions get made oblivious of customer perception of products and services? Why do good sale executive make sales managers because of their sales success? Why is an accountant made a SKU manager? Just because you can talk to people does not qualify you to interview candidates? Why is an entrepreneur also the CEO. And all of these without the benefit of proper evaluation, training, and transition.
To a naïve eye a swing of the golf club by a pro and a novice may seem the same but not to the trained one. So, what are some of the practices you follow that you do so oblivious of the fact that there are methods out there to handle them correctly.
The consequence of training, or lack thereof, are not insignificant. Remember, nothing hides mistakes like success does.