Right after you take heroin, you get a rush of good feelings and happiness. Then, for several hours, you feel as if the world has slowed down. You think slowly and may walk slowly. Some users say you feel like you're in a dream.

https://harmreduction.org/issues/overdose-prevention/overview/overdose-basics/opioid-od-risks-prevention/ Both morphine and 6-MAM are μ-opioid agonists that bind to receptors present throughout the brain, spinal cord, and gut of all mammals. The μ-opioid receptor also binds endogenous opioid peptides such as β-endorphin, leu-enkephalin, and met-enkephalin. Repeated use of heroin results in a number of physiological changes, including an increase in the production of μ-opioid receptors (upregulation). [81] These physiological alterations lead to tolerance and dependence, so that stopping heroin use results in uncomfortable symptoms including pain, anxiety, muscle spasms, and insomnia called the opioid withdrawal syndrome. Depending on usage it has an onset 4–24hours after the last dose of heroin. Morphine also binds to δ- and κ-opioid receptors. Dahlin U (February 2009). "Heroin-behandling bliver kun i kanyler"[Heroin treatment stays only in needles] (in Danish). Information. Archived from the original on 20 February 2009 . Retrieved 14 February 2010. A person can overdose on heroin. Naloxone is a medicine that can treat a heroin overdose when given right away, though more than one dose may be needed.Physical dependence means the body adapts to the presence of the drug. Withdrawal symptoms also occur if use is suddenly stopped.

Nutt DJ, King LA, Phillips LD (November 2010). "Drug harms in the UK: a multicriteria decision analysis". Lancet. 376 (9752): 1558–65. CiteSeerX 10.1.1.690.1283. doi: 10.1016/S0140-6736(10)61462-6. PMID 21036393. S2CID 5667719.

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Heroin overdose is usually treated with the opioid antagonist naloxone. This reverses the effects of heroin and causes an immediate return of consciousness but may result in withdrawal symptoms. The half-life of naloxone is shorter than some opioids, such that it may need to be given multiple times until the opioid has been metabolized by the body. The Health Committee of the League of Nations banned diacetylmorphine in 1925, although it took more than three years for this to be implemented. In the meantime, the first designer drugs, viz. 3,6 diesters and 6 monoesters of morphine and acetylated analogues of closely related drugs like hydromorphone and dihydromorphine, were produced in massive quantities to fill the worldwide demand for diacetylmorphine—this continued until 1930 when the Committee banned diacetylmorphine analogues with no therapeutic advantage over drugs already in use, the first major legislation of this type. [ citation needed] Brecher EM, etal. (Editors of Consumer Reports Magazine) (1972). "Chapter 12. The "heroin overdose" mystery and other occupational hazards of addiction". The Consumers Union Report on Licit and Illicit Drugs. Schaffer Library of Drug Policy. Archived from the original on 8 February 2007. Eason K, Naughton P (13 March 2012). "First murder charge over heroin mix that killed 400". Times Online. London . Retrieved 20 July 2012. Methadone, an opioid receptor full agonist, attaches to and activates opioid receptors to ease withdrawal symptoms and cravings.