You're right, it's not 100% reliable. Yet.

But it's getting there. And It's getting there fast.

DNA tests, if totally reliable, will take much of the quess-work out of prescribing medication that will both work and will have as few side effects as possible.
Here's How:
It's easy to get your DNA by swabbing the inside of your mouth with a q-tip.
Let’s say your DNA happens to be the highly improbable

1 2 1 2 1 2 1 2 3 4 5 6 7 8 9 0
By the way, This is just an example drawn out of thin air.
Well, suppose tests show that people who share the DNA sequence 1 2 3 4 5 6 7 8 9 0 don’t breakdown aspirin as quickly as most.
So you should take less aspirin than most people.

Even more extreme, let’s say that other people with your DNA sequence (1 2 1 2 1 2 1 2)
actually can’t even tolerate aspirin. Or it doesn’t do a thing for them. Then you don’t take it.

You can see how this technique might be used in prescribing drugs. The sequencing might be harder to find, but DNA testing is a step in the right direction. After all, if, according to your D.N.A., aspirin (or a prescribed drug) won’t even help you, or could even hurt you, you will avoid it.

Linnie