Is there any evidence to suggest that the war on drugs in the Philippines by president Duterte has proven to be successful, with regards to the reduction of drug-related crimes?
As indicated by comments, it is very hard to evaluate the effectiveness of the fight against war on drugs. The official data is presented on Wikipedia, but they are challenged by other sources:
Illegal drug use and trade: - Proliferation of drugs: 4 million drug addicts, estimated - Illegal Drug Market: ₱120 billion drug industry, estimated
War on drugs: - Police operations conducted: 53,503 - People arrested: 64,917 - Value of seized drugs and non-drug evidence: ₱14.49 billion - Killed in legitimate police operations: 2,679 - Drug pushers voluntary surrendered: 88,940 - Drug users voluntary surrendered: 1,266,966 - Reduction in drug market: 26.45%
This article dives into more aspects of the war on drugs.
Undervaluation of legitimate killings:
Nearly 2,300 people have been killed in police operations or by suspected vigilantes since Duterte took office on June 30, according to the Philippines police. That figure was revised down this month by the police from an original tally of 3,600 deaths.
Overestimation of number of drug addicts
In his inaugural State of the Nation Address on July 25, Duterte declared that there were 3.7 million “drug addicts” in the Philippines.
“The number is quite staggering and scary,” he said. “I have to slaughter these idiots for destroying my country.”
But according to a 2015 survey by the Office of the President’s Dangerous Drugs Board (DDB), the main drug policy and research unit, the Philippines has fewer than half that many drug users.
Biased means of addiction assessment
Joanne Csete, a specialist in health and human rights at the Mailman School of Public Health at Columbia University in New York, said that the term “current drug users” usually refers to those who have used drugs in the past month. However, the DDB survey counts anyone who has used drugs in the past 13 months, which Csete says could inflate the number of users.
Biased interpretation of serious crimes decrease and an actual increase in murder rate
“Index” or serious crimes in the Philippines dropped by 31 percent in January to August this year compared with the same period in 2015, according to police statistics presented to a Senate hearing on extrajudicial killings on Oct. 5.
But the same police statistics show serious crime was already in decline during the administration of Duterte’s predecessor [...]
While the crime rate has been dropping for several years, under Duterte the murder rate has risen since he launched his anti-drug campaign.
Human Rights Watch investigates even more the war on drugs and uncovers other aspects.
Human Rights Watch found that the official police reports of these incidents invariably asserted self-defense to justify police killings, contrary to eyewitness accounts that portray the killings as cold-blooded murders of unarmed drug suspects in custody. To bolster their claims, the police routinely planted guns, spent ammunition, and drug packets next to the victims’ bodies.
President Duterte has frequently characterized his “war on drugs” as targeting “drug lords” and “drug pushers.” However, in all but one of the cases investigated by Human Rights Watch, the victims of drug-related killings by the police or unidentified gunmen were poor (the exception was a middle-class victim who appears to have been killed as a result of mistaken identity), and many were suspected drug users, not dealers at all. Almost all of the victims were either unemployed or worked menial jobs, including as rickshaw drivers or porters, and lived in slum neighborhoods or informal settlements.
Public health consequences
Duterte’s War on Drugs [...] likely to have significant negative public health consequences. Human Rights Watch has documented in various countries that harsh drug enforcement can lead to drug users going underground away from critical health services.1 This can fuel the transmission of HIV and Hepatitis C among people who used drugs and may discourage people with drug dependence from seeking effective treatment services.
I cannot provide a direct answer to the question, since success cannot be measured based on achievements only. The cost also matters.
Considering all above, it is very hard to estimate whether the war was successful or not (do the benefits outweigh the cost?). Even if the drug market is significantly reduced, the killings cannot be just ignored. Even if drug related crime if reduced, crime rate has increased and this is partly related to vigilante killings.