Heroin users in Barnsley are injecting MCAT instead because it is more available, according to drug charity Addaction.
James Pierce, of the harm reduction team at Addaction, said there had been a switch from the injection of heroin to users experimenting with the intravenous use of mephedrone (MCAT), which was made illegal in 2010.
He said Barnsley was one of the first areas in the UK to begin to see people injecting the drug in 2012.
It comes after it was announced by police that two thirds of all seizures of MCAT in the county last year were made in Barnsley.
Of the 252 kilos of the drug, which is made using plant fertiliser, recovered, 161kg came from Barnsley.
James said: "We still see large numbers of heroin users, but we're seeing increasing numbers of people injecting steroids and have seen a real shift in people injecting mephedrone.
"The increase is generally down to availability and the perceived low strength of other street drugs such as cocaine, ecstasy and heroin.
"Police seizure figures for Barnsley show there are large amounts of mephedrone available and this has led to high levels of use."
James said the compulsive nature of the drug meant that when it is injected dependency becomes more likely.
People who inject may experience a negative impact on their mental health and put themselves at risk of injecting injuries and other associated problems.
But James also added there was a decline in the number of people using heroin and could perhaps be down to successful treatment and people choosing not to use drugs.
"There is an ageing population of heroin users. We don't see many young people using heroin, which is really positive."