The Unpredictable Nature of Today's Synthetic Drugs
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These days, if you take synthetic drugs, what exactly will happen to you is anybody's guess.

In the city of Newcastle in England, for example, a synthetic drug called "Insane" has people acting like "zombies," according to The Independent. Residents who have taken the drug have experienced paranoid delusions, fits of "body popping" or uncontrolled muscle movements — and sometimes — sudden collapses, according to the report.

Synthetic drugs are a huge class of drugs that are generally designed to mimic the effects of illegal drugs including amphetamines or ecstasy, but they have a chemical tweak or two that keeps them from being illegal. To make them, sellers usually spray man-made chemicals onto plant material.

The use of these drugs — which will become illegal in the United Kingdom with a broad new law that goes into effect on May 26 — isn't limited to the city of Newcastle.  [The Drug Talk: 7 New Tips for Today's Parents]

Synthetic drugs are widespread, said Dr. Simon Thomas, a consultant pharmacologist at Newcastle Hospitals NHS Foundation Trust in England. And more than 500 different drugs have been found in Europe, he said.

The mix of chemicals in the drugs is constantly changing, however. For example, recently, the most common substances found in the drugs have been synthetic chemicals that are similar to the active ingredient in marijuana, tetrahydrocannabinol (THC), Thomas said. THC is a complex molecule, and it's very easy to make small changes to different parts of it and produce a very large number of different substances that have different effects, he said.

These effects may be similar to — but are not identical to — those of marijuana, he said.

They're sold as "herbal substances" that users can smoke, but in fact they've got synthetic chemicals sprayed onto them, he added.

Because people make hundreds of different products with slightly different chemicals, but then sell them all in similar packaging, it's very hard to know precisely what's in any one package at any time, Thomas told Live Science.

Indeed, part of the risk associated with using these substances is that you don't know what you're getting, Thomas said. A person may try a product they've used before and enjoyed, but the second time, it could make him or her ill, he said. The reason for this is that even though it comes in the same packaging, it could contain a different chemical. Or the product could have the same chemicals, but in different amounts. For example, chemicals are not always sprayed on the plants evenly, so a person could end up ingesting a highly concentrated amount, he said. 

Synthetic drugs are not regulated, and so just because you can buy a product in a store doesn't mean it's safe, he said. And not knowing what exactly you're ingesting can make it difficult for you to anticipate the effects.  

Synthetic marijuana, for example, may have effects that are similar to the real thing, but it can also produce effects that are not similar to cannabis, and those can be quite hazardous, Thomas said.

The compounds may cause loss of consciousness, seizures and severe behavioral disturbances including aggression, paranoia and psychosis, Thomas said. The drugs have also been linked to kidney failure, he added.  [11 Odd Facts About Marijuana]

In addition, the drugs may be addictive.

Users have reported that they have had difficulty stopping using the drugs, Thomas said.  

What's going on in the United States?

The United States faces problems with these "new psychoactive substances" — as they are sometimes called — that are similar to those in the U.K.

Although the U.S. Drug Enforcement Administration bans existing formulas of many drugs, new, tweaked versions often emerge on the market, according to Consumer Reports.

As in the United Kingdom, the effects of these new substances are unknown and potentially dangerous.

Indeed, between January 1, 2016, and April 30, 2016, there have been over 1,000 calls to poison control centers related to synthetic marijuana, according to the American Association of Poison Control Centers.

Follow Sara G. Miller on Twitter @saragmiller. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.