Officials in a Pennsylvania town say they treated 65 people who overdosed on synthetic marijuana in just one weekend.
Although none of the overdoses were fatal, first responders received dozens of calls.
There are many types of synthetic cannabinoids – one being synthetic marijuana – chemicals that have been developed to act like the main psychoactive chemical in cannabis, tetrahydrocannabinol (THC).
THC acts on cannabis receptors in the brain and affects a person's memory, movements, thinking, concentration and pleasure.
The effects of synthetic marijuana are similar to cannabis – so some people will feel happy and relaxed, some hungry and some paranoid or sick.
However, synthetic marijuana can be more potent as it reacts more strongly with the brain's cannabis receptors – which means it is easier to overdose and experience negative side effects.
The potency of synthetic cannabinoids means they are more likely to be associated with hallucinations.
According to the US National Institute on Drug Abuse, synthetic marijuana – often marketed as K2 –can cause kidney damage and seizures. It can also raise a user's blood pressure and caused reduced blood supply to the heart.
There may also be unknown effects too because many synthetic cannabinoids are new.
In the UK, many synthetic cannabinoids are illegal.