MSP Graeme Pearson spoke out as health bosses claimed only EIGHT people had been admitted to city hospitals because of the deadly substances in the past four years.
Mr Pearson said he was shocked by the low figure and branded it "irrational".
The founder of the Scottish Drug Enforcement Agency said: "Until we know the true scale of the problem, we don't know how to deal with it."
His comments came after the Evening Times reported this week there had been an 800% surge in the number of legal high deaths in the UK.
Legal highs are substances that produce similar effects to illegal drugs but are chemically different enough to not be covered by the law. They are on sale openly on websites and high streets across the UK.
Mr Pearson said: "All the other evidence shows a large number of people are taking these substances and many are suffering medical problems.
"More than 40 people died in 2012 because of so-called legal highs. So it hardly seems logical that only eight people would be admitted to hospital over a four-year period.
"Only now are people beginning to realise these substances can cause deaths. And we need to get our heads around what is in them."
People who have taken the drugs can suffer severe sickness and heart palpitations, as well as hallucinations and anxiety.
Mr Pearson has called for more "logical" recording of people who are admitted to hospital suffering from the effects of new psychoactive substances.
NHS Greater Glasgow and Clyde's data, released to the Evening Times under Freedom Of Information legislation, says eight people were admitted to hospitals for emergency treatment because of legal highs between 2008-09 and 2012-13.
The exact number of people each year was withheld to protect their identification.
Mr Pearson added: "I understand medics have more to worry important things to worry about and they are focusing on the immediate challenge.
"But unless we know the true size of the group of people using these substances, we do not know what we are dealing with.
"Nobody knows what these substances are and what they contain."
Earlier this week, researchers warned people who take legal highs were "dancing in a minefield" after an 800% surge in the number of deaths, from 12 in 2009 to 97 in 2012.
The figures were published in the National Programme on Substance Abuse Deaths report. In many cases studied by the programme, traces of multiple legal highs were found, suggesting drug users are experimenting with combinations of these drugs.
Mr Pearson has previously warned legal highs and ecstasy tablets are becoming "as dangerous" as hard drugs, such as heroin and cocaine.
This month 17-year-old Regane MacColl fell ill in Glasgow's Arches nightclub and died on February 2 in the Royal Infirmary. Police believe her death could be linked to red Mortal Kombat ecstasy tablets.
The pills, which have a dragon stamped on them, were also linked to four other cases of people falling ill on the same weekend the schoolgirl died.
An NHS Greater Glasgow and Clyde spokeswoman said: "Our systems record the primary or main reason why a person is admitted to A&E.
"We often rely on the person being admitted to A&E or those accompanying them to provide information on any drugs ingested.
"In some instances we may be made aware by the person attending A&E that they have taken a legal high.
"However, the reason for the attendance will normally be the outcome of taking these chemicals, such as, vomiting, dizziness or physical harm.
"As this is the condition that needs treatment this is what would be recorded on our systems."