Activists will gather outside parliament on Wednesday to press for the passage of the cannabis control bill, which is currently having its second reading.
Their leader, Prasitchai Nunual, said they want parliament to enact the law to prevent the liberalisation of cannabis use and were opposed to some MPs threatening to vote against it.
"If any MPs attempt to block the passage of the bill, people will give them a painful lesson [by not voting for them] in the coming election," he said.
Sonthaya Sae Yo, a traditional medical practitioner in Surat Thani's Koh Phangan district, said he has used cannabis to treat numerous patients.
However, he said he disagreed with the liberalisation of cannabis use and that it was necessary to enact the bill to control it.
"What is worrying is that opponents of the bill are trying to vote against it for their own political gain," he said.
The bill was already vetted by a House committee comprising all stakeholders, Mr Sonthaya said, adding that some had spread false information about minors smoking cannabis.
The Democrats have been critical of the Bhumjaithai Party, suggesting legal controls over the use of cannabis and hemp, as stipulated in its bill, may be lax.
Dr Banyat Jettanachan, a Democrat MP from Rayong, told a press briefing on Dec 2 that it is uncertain when the cannabis control bill will be passed by parliament as reviewing proposed legislation takes time.
He also referred to a photo posted on Facebook on Nov 26 by a Pattaya local allegedly showing two 10-year-olds smoking cannabis on a beach during a festival.
The photo went viral, sparking a heated debate over whether it was appropriate for children to have access to cannabis.
The issue of cannabis control predates the bill.
Removing cannabis and hemp from the list of narcotics was made possible by a Public Health Ministry announcement that came into effect earlier this year.