Even the people dealing the drugs don’t know what’s in them in most instances

I used a book called The Economics of Public Issues as a supplemental text. One chapter was called "Sex, Booze and Drugs." It mentions how buyers won't know what is in an illegal drug since the seller has little incentive to tell them or label the ingredients. If there is something bad in the cocaine or heroin, who will the buyer tell? The police? If so, they would be admitting to a crime.

See By Noah Weiland of The NY Times. Excerpts:

"Erin Tracy, a chemist at the university who specializes in testing drugs, began searching for the answer. She dispensed the sample into a small vial, then loaded it into a $600,000 refrigerator-size device known as a gas chromatograph-mass spectrometer, which is commonly used in academic chemistry laboratories. A nearby computer displayed the results in a line graph with a dramatic peak — the signal for fentanyl."

"The testing of drug samples at the lab and in a growing number of cities across the country is delivering new insight to researchers and drug users about what is in the local drug supply. Drug users can learn what is in a substance before they use it, alert other users to possible dangers in the supply or find out why a drug led to an overdose or some other reaction. The Chapel Hill team has also examined samples of drugs that caused fatal overdoses and then relayed the results to harm reduction groups.

The testing work in North Carolina and elsewhere, known as drug checking, has become especially critical to recognizing fentanyl, a synthetic opioid and a chief culprit in many overdose deaths in recent years. Other drugs, such as cocaine and heroin, are often mixed with fentanyl.

But the U.S. drug supply is increasingly being contaminated by other substances as well, such as xylazine." 

"people who were fatally overdosing from drugs were not just dying from fentanyl, but also from other contaminants."

"In more than a dozen states, even the basic tools of drug checking, such as fentanyl test strips, are outlawed as drug paraphernalia"

"Users are often left to intuit what they take based on a pill or powder’s smell or appearance, such as its lightness or darkness. Samples from the same drug dealer can contain different amounts of fentanyl, with other substances mixed in that might vary from dose to dose. And the effects of drugs can differ from batch to batch, which can have trace amounts of substances that sometimes cause odd and surprising sensations."

"Unlike some drug checking machines, the strips do not tell a user the variety or amount of fentanyl in a sample — only whether it contains the drug."

"Tools for drug checking “create an early warning system that simply is never going to happen from D.E.A. drug seizures in a criminalized drug supply,” Mr. Ferguson said. “Even the people dealing the drugs don’t know what’s in them in most instances.”"