Jo Swinson has claimed the Liberal Democrats' policy to legalise cannabis will create additional revenue for the UK and reduce crime on the UK's streets.
Speaking to ITV News Correspondent Rebecca Barry, Ms Swinson said current drugs laws "just aren't working" and reform was needed to ensure Britain doesn't "lag behind" other countries which had made cannabis legal.
The policy was one of a number of sweeping reforms included in the Liberal Democrat manifesto, which was released on Wednesday.
Ms Swinson said: "We're criminalising far too many young people. It's also meaning that it becomes a gateway to other crime.
"If instead we can regulate the cannabis market, we can raise taxation revenue from that, we can set regulations about the safety of the cannabis which is bought and sold.
"Then we can create a much safer environment. This is something that is led by the evidence.
"It's happened in other countries where they've seen a decrease in crime rate and indeed they've been able to make sure people's health has been protected too."
Ms Swinson rejected criticism that the legalisation of cannabis could negatively impact mental health and mental health services in the UK, saying the deterrent of cannabis being illegal at the moment was not having an affect on users.
"There's a range of substances which absolutely have negative impacts on our health but are nonetheless legal, and adults are in a position to decide whether they will go and buy a drink at a bar, whether or not they will smoke," Ms Swinson said.
"Just the fact of it being illegal isn't stopping people from doing it. The surveys are very clear that it's a very, very widely used drug.
"But criminalising people for something which many people in society do not see as being a dreadful thing to do doesn't seem to make sense."
In a wide-ranging interview, Ms Swinson also advocated for reforming stop and search powers of police, adding she wanted to pursue "policies that work" when it comes to reducing knife crime.
The Liberal Democrat leader said she advocated a "joined up approach" involving police, health care and youth agencies working together to tackle the problem.
Ms Swinson added the Liberal Democrats would increase the number of police to tackle crime and those most at risk, adding she would also provide additional funding for youth services to help tackle the issue.
She has today claimed there would be a £50bn "Remain bonus" of staying inside the EU.
She said the money would bankroll Lib Dem spending plans and defended the figure, claiming it has been supported by economists.
"There is wide agreement from economists that if we leave the EU, we will be poorer," she said.
"If we stay in the EU, there will be billions of pounds more to spend on our public services, on our schools, in helping people get out of in work poverty, so its important that we set that out."
Watch Rebecca Barry's interview with Jo Swinson here: