世界衛生組織藥物依賴專家委員會指出，「藥物濫用」即「持續或偶爾用藥過量，而此用藥行為與治療無關」。若以行為角度來看，藥物濫用是指：
1.
 不論是藥物種類、服用方式及地點均不合宜；
2.
 在欠缺醫生指引下過量用藥，
3.
 濫藥者不能自拔，有強迫性用藥行為；
4.
 導致精神及身體受損，危害社會。
香港政府於1972年設立藥物濫用中央檔案室(CRDA)。三十多年來，濫藥者數目一直在13,171至20,328之間，看似變動不大。其實主因在於被呈報的都是被捕或尋求戒毒服務者。以下是歷年的濫藥走勢。
2008年CRDA的數字顯示，在香港最常被濫用的藥物依次是：海洛英(51.2%)或稱「白粉」、氯胺酮(35.6%)、甲基安非他明(冰毒)(9.6%)、三唑侖/咪達唑侖/佐匹克隆(9.5%)和搖頭丸(MDMA)(5.6%)。海洛英在香港數十年來一直是最流行的毒品，濫用海洛英者不少以打針方式注射藥物，因而有機會感染經血液傳播的疾病，如HIV及丙型肝炎等。
 
According to the WHO Expert Committee on Drug Dependence of the World Health Organization, drug abuse is defined as "persistent or sporadic excessive drug use inconsistent with or unrelated to acceptable medical practice". In terms of behavior, drug abuse covers:
 
1.
 the types of drugs used, how the drugs are taken, or the venue of drug use are not acceptable;
2.
 self medication without supervision by a physician, outside the medical boundary and the standard dosage;
3.
 the drug abuser loses self control and takes drugs compulsively;
4.
 the drug abuse causes physical, psychiatric or social harms.
In Hong Kong, a Central Registry on Drug Abuse (CRDA) was set up in 1972. The annual figures since 1976 indicated the number of reported drug abusers in Hong Kong all through the years were quite stable: between 13,121 to 20,328. The reason for the low fluctuation was that only those who were arrested or who sought treatment were counted. Anyway, the trend of drug abuse can be observed.
According to the 58th CRDA report, in the year 2008, the five most common drugs of abuse in Hong Kong were Heroin (51.2%), Ketamine (35.6%), Methamphetamine (Ice) (9.6%), Triazolam/Midazolam/Zopiclone (9.5%), and MDMA (5.6%). Heroin has been popular in Hong Kong for decades. One common way of abusing heroin is through injection, which may bring blood borne diseases like HIV or Hepatitis C to the drug abuser.