Five students from Lancaster University have been taken to hospital after using a synthetic cannabis substitute known as "spice".

The university sent out an alert on its Twitter account shortly after 9am on 20 May asking students to check on friends and call for an ambulance if any were unwell. Three students have been released, but two remain in hospital in "critical condition".

"Spice" refers to a wide variety of herbal mixtures known as synthetic cannabinoids.

"Due to the serious condition of the students and the fact the substance could not be confirmed as legal, officers attended the university to carry out a search of the students' rooms," Lancaster University told the Guardian in a statement. "Inquiries are ongoing to identify the exact nature of the substance."

What is spice?

Synthetic cannabinoids are chemicals that are formed to act like the active part of cannabis, a substance called tetrahydrocannabinol (THC). These products contain dried and shredded plant material and chemical additives which are responsible for their psychoactive effects.

There are many different types of synthetic cannabinoids and a large number have become Class B drugs, which are illegal to have or sell. According to the drug helpline Frank, it is difficult to tell what is in a smoking mixture so it is hard to tell if a substance is legal or not.

Other names for synthetic cannabinoids include K2, fake weed, Yucatan Fire and Moon Rocks.

What are the risks of using synthetic cannabinoids?

The substances have relatively unknown effects because they are so new, and may be stronger than typical cannabis. Some can make you feel happy and relaxed, others may change mood, perception and co-ordination.

Other users have reported bad reactions such as paranoia, an increased heart rate, sickness, panic attacks and hallucinations.

The effects of some synthetic cannabinoids may be longer lasting than cannabis and because of the lack of information about what is in them, it is difficult to predict the strength of the product.

"Because synthetic cannabinoids act like THC, the main active ingredient in cannabis, it is very likely that they will produce harmful effects similar to those associated with cannabis," the Frank website states.

"We know that there have been a number of deaths that have been related to the use of synthetic cannabinoids, either on their own or with other substances."

Some reports about the synthetic cannabinoid AM2201 suggest it can cause increased heart rate, panic attacks and convulsions, so it is likely other similar smoking mixtures have similar adverse effects.