Synthetic marijuana compounds, sometimes called K2 or Spice, are actually chemically different from marijuana, and are more dangerous than pot, according to a new review of studies.

Synthetic marijuana compounds are linked to a number of serious side effects, including seizures, psychosis and even death, the review's authors said.

These compounds "produce a variety of dangerous acute and chronic adverse effects … with a greater severity and frequency than observed following marijuana use," the researchers wrote in their review, which was published Feb. 2 in the journal Trends in Pharmacological Sciences. Therefore, "K2/Spice products are clearly not safe marijuana alternatives," the researchers said.

Manufacturers first started to sell synthetic marijuana compounds in the early 2000s. The products are often marketed as "safe" alternatives to marijuana that will not show up on a standard drug test.

In the early 2000s, "we started seeing all kinds of people coming into emergency rooms saying they smoked marijuana, but then they had these really bizarre symptoms that did not correspond with the effects you see with marijuana," Paul Prather, a cellular and molecular pharmacologist at the University of Arkansas for Medical Sciences (UAMS), said in a statement.

There are now more than 150 different types of synthetic marijuana compounds, the researchers said. The products are designed to activate two receptors in the body, called CB1 and CB2 — the same receptors that tetrahydrocannabinol (THC), the active ingredient in marijuana, binds to. But although they activate the same receptors, synthetic marijuana compounds are structurally different from marijuana, the researchers said. In addition, as a group, these compounds have diverse chemical structures, meaning that they are often quite different from each other as well, they said.

Many types of synthetic marijuana compounds are illegal in the United States, but clandestine manufacturers continue to develop new compounds that aren't always covered by existing laws and aren't always detected by current drug tests, the researchers said. [9 Weird Ways You Can Test Positive for Drugs]

Some of the reported side effects of synthetic marijuana compounds include convulsions, kidney injury, toxicity to the heart, strokes and anxiety. So far, 20 deaths have been linked to the use of synthetic marijuana compounds, the researchers said.

Studies have found that these compounds tend to activate the CB1 receptor to a greater degree than does THC, suggesting that synthetic marijuana compounds have the ability to induce far more intense effects than marijuana, according to the review.

In addition, when some of these compounds are broken down in the body, their by-products also have the ability to activate the CB1 receptor, which could contribute to the increased toxicity of the drugs, they said. What's more, because of their diverse structures, synthetic marijuana compounds may also activate other receptors besides CB1 and CB2 — which could explain why these drugs produce some adverse side effects that are not seen with marijuana, they said.

The researchers also note that when people purchase synthetic marijuana compounds, they don't know what's really in the product that they are buying.

"Not only does the amount of the active pharmacological agent change with different batches of drugs, made by different labs, but the active compound itself can change," said William Fantegrossi, a behavioral pharmacologist at UAMS. And there are usually a minimum of three different types of synthetic cannabinoids in a single product, the researchers said.

More research is needed to determine exactly why synthetic marijuana compounds appear to be more toxic to people than marijuana, the authors of the review said.