There are sites where people can buy controlled substances on the Internet. True controlled substances, only in unlimited amounts and without doctor’s prescriptions. While they might contain real products, the buyers could potentially use them indiscriminately, OD and die. Does it fall under “tampering”?
No.
Tampering is deliberately altering products or goods with the intent of harming people. Think of someone mixing metal shavings into a food product that is sold by a grocery store or finding a way to mix cyanide into an over the counter medicine that is then bought by consumers. Your example would only be tampering if, after purchasing said controlled substances the buyer mixed them, unknown to customers, into a product like Tylenol or toothpaste, for example, where no one expected it to be.
I think the question you're asking is if someone bought "black market" codeine, for example, and someone took it and overdosed and died, did the person who provided the drug "tamper" with codeine? I'd say no, they just allowed someone to indiscriminately use a dangerous substance that they should not have been in possession of. I think this would be charged under drug laws, not tampering laws. MOO