I think that the variation in people needs to be addressed.
I love what Dorian Yates said "Steroids are like smoking, there are some people who are still puffing away at 90, and there are others who have lung problems at 40" (or something to that effect).
I have been reading this book called Biochemical Individuality, and it has been incredibly interesting and a real eye opener.
Not all the enzymes we have in our body function with exactly the same efficiency in every single person.
There is also a huge variation in just the SIZE of the liver, so if you were someone who was born with a smaller liver......................
This is what I do know about the pharmacology of drugs, they are processed in the liver in the mitochondria in the p450 cytochrome system.
Mitochondria are funny little organelles, it is thought that they, and chloroplasts (the similar energy making machinery in plants), arose because they were more or less 'eaten' by a bigger single cell organism in the primordial times. They have a structure and DNA that is more similar to bacteria (prokaryote), than a typical animal cell (eukaryote).
We get our mitochondria from our moms only, and as I mentioned, they have a completely different set of DNA than the rest of our body cells.
With regards to drug processing, everything we eat makes a first pass through the liver and is metabolised, and a lot of drugs are designed to be able to activated here, or withstand metabolism etc (pharmacology is seriously complicated).
Not all drugs work on everyone due to physiological and biochemical variations, some drugs have a detrimental effect on people (adverse events), some drugs will not work on certain 'racial' groups.
While it is great that you have not had any bad effects (although chazK also has a point about tumours being 'walled off'), you can't say that they will be safe for everyone.