What do they do with drugs as a piece of evidence?
Each jurisdiction has their own set of rules, regulations, and laws, but generally the drugs will be disposed of or they might be kept for training or future investigative operations.
Drugs are destroyed after they are no longer needed for legal proceedings. In the United States, the Drug Enforcement Administration (DEA) burns illicit drugs at contract facilities. Local law enforcement may use incinerators at hospitals, veterinary clinics, crematories, foundries, hospital incinerators, labs or specialized businesses (Drug destroyer agencies) and may even torch drugs in 55-gallon drums. U.S. Customs and Border Patrol, which seizes millions of pounds of illegal narcotics, pays contractors to destroy the drugs. At times small amounts of drugs may be flushed down the toilet. Canadian officials sometimes render drugs unusable by mixing them into a plaster that is later buried.
Some countries may follow the internationally accepted destruction methods as per United Nations Office on Drugs and Crime.