Labour and the Greens oppose the bill, saying it will do nothing to reduce harm.
Brown revealed National's support for the NZ First change on Wednesday as the committee stage of the bill was debated in the House.
"When I first put this bill in the ballot ...we could see that there were people dying from these drugs, and we put it in the ballot as a Class C," Brown told the House.
"Since then we've seen the coroner's report - 45 deaths from synthetic drugs over the past 12 months, and more and more happening every single week.
"Parliament needs to send a very clear message that if you are going supply and manufacture and distribute these substances, then they will face the full force of the law."
He said the bill would apply to every concoction that synthetic drug dealers come up with.
Justice Minister Andrew Little told the House that the bill offered no legit solution to the harm that drugs do to society.
"Simeon Brown makes a very good point, which is that he considers that his party, by passing this bill, is not standing by while people die. In fact, by passing this bill, that is exactly what this House is doing.
"Anybody with experience and expertise who has looked at this issue and looked at this bill has been very consistent in saying that threatening longer penalties isn't going to change people's behaviour."
Little said the law had the tools to properly prosecute peddlers of synthetic drugs.
"The reality is that if administering a toxic substance like this is causing death, then that, actually, is prima facie an allegation of manslaughter. We can charge people for those sorts of crimes right now, even without this legislation."
He said the Government had a broader plan involving the justice and health sectors to address the underlying causes of drug use.
"If this House is serious, and members in this House are serious about their moral duty to every New Zealander to make our communities safe and safer, we wouldn't be wasting our time on silly gestures like this."
The bill will continue to progress on the next Members' Day, which is November 28. It is likely to pass its final reading before the end of the year.