Metro Manila (CNN Philippines) — The former President of a country notorious for its illegal drug cartels is warning President Rodrigo Duterte that his war on drugs will likely backfire.

"Throwing more soldiers and police at the drug users is not just a waste of money but also can actually make the problem worse," said former Colombian President Cesar Gaviria in a Tuesday opinion piece in The New York Times.

"Locking up nonviolent offenders and drug users almost always backfires, instead strengthening organized crime," he added. "That is the message I would like to send to the world and, especially, to President Rodrigo Duterte of the Philippines. Trust me, I learned the hard way."

Gaviria — who was President of Colombia from 1990 to 1994 and was involved in taking down international drug trafficker Pablo Escobar — said using brute force in combating illegal drugs just created more problems.

"Not only did we fail to eradicate drug production, trafficking and consumption in Colombia, but we also pushed drugs and crime into neighboring countries," he said.

"Tens of thousands of people were slaughtered in our anti-drug crusade," Gaviria added. "Many of our brightest politicians, judges, police officers and journalists were assassinated. At the same time, the vast funds earned by drug cartels were spent to corrupt our executive, judicial and legislative branches of government."

According to Philippine National Police (PNP) data, more than 2,500 alleged drug suspects were killed in police operations from July 1, 2016 to January 30, 2017.

Meanwhile, human rights group Amnesty International said it has recorded over 7,000 drug-related killings, alleging that police officers are paid up to ₱15,000 for every drug suspect killed.

Read: War vs. poor: Police paid per drug killing – Amnesty International

Other human rights groups and members of the international community have condemned Duterte's war on drugs because of incidents of alleged extrajudicial killings.

Related: Report: Philippines on downward trend in upholding freedom due to war on drugs

The government has repeatedly denied the allegations, saying the deaths were a result of police officers defending themselves in legitimate police operations.

In addition, a Senate investigation found no evidence of extrajudicial killings sanctioned or committed by authorities.

Related: CHR: Senate should continue probe on extrajudicial killings

Hard line stance 'unwinnable'

Gaviria also said taking a hard line stance against criminals may be popular for politicians, but the human costs were enormous.

"I was also seduced into taking a tough stance on drugs during my time as president," he said. "The polls suggest that Mr. Duterte's war on drugs is equally popular. But he will find that it is unwinnable."

"They started making positive impact only when they treated drug use a social problem and not a military one." he added.

New approach to dealing with drugs

Gaviria also said the war on drugs is a "war on people" and that more and more governments see the need for a new approach to dealing with illegal drugs.

"Real reductions in drug supply and demand will come through improving public health and safety, strengthening anticorruption measures — especially those that combat money laundering — and investing in sustainable development," he said.

Gaviria also said the best way to tackle drugs is to decriminalize consumption and ensure that governments regulate certain drugs, including for medical and recreational purposes.

The Duterte government said it has committed funding for drug rehabilitation programs and has likewise tapped private investors to put up drug-rehabilitation facilities.

Read: Duterte admin to focus on rehab of drug dependents

Gaviria also said Duterte's idea of using the armed forces in the war on drugs would be "disastrous."

The proposal comes after Duterte ordered the PNP and the National Bureau of Investigation to suspend anti-drug operations following the kidnapping and killing of Korean businessman Jee Ick-joo in a purported anti-drug operation.

Read: War on drugs suspended, takes back seat in war against rogue cops

"No matter what Mr. Duterte believes, there will always be drugs and drug users in the Philippines," Gaviria said. "But it is important to put the problem in perspective: The Philippines already has a low number of regular drug users. The application of severe penalties and extrajudicial violence against drug consumers makes it almost impossible for people with drug addiction problems to find treatment. Indeed, the criminalization of drug users runs counter to all available scientific evidence of what works."

Malacanang has yet to comment on Gaviria's statements.