I think the joke is that it's a rating rather than a sample.

So this doctor gets 1* reviews vs the 4.5* doctors.

I had the same initial reaction but I think the doctor is treating it as a rating and not a fraction. E.g. a 90% grade doctor would recommend it, but they're a 20% doctor.

It's a different joke. He's saying a doctor with good ratings like 9/10 would say to stop, but he has bad ratings, 2/10, so he's going to give bad advice.

When reading the joke, replace the / with "out of" and the wording comes out better for the joke.

9 out of 10 doctors would suggest you quit. Luckily I'm a 2 out of 10 doctor.

Therefore good doctors would say you should quit, but I'm not a good doctor.

I would make less sense because then you would think he's saying 9 out of 10 different doctors recommend against it, ruining the punchline.

He should have said 1/8 doctor

I have never heard of "2/10", but after a bit of research, I see a couple very loose connections:

• An accounting term regarding a discount for prompt payment. I do not see reference of this being used in a drug context
• Smoking something and rocking the pipe back and forth between the "2" and "10" positions on a clock. I do not think this applies to cocaine, or even "crack"