First-time users frequently experience unpleasant, or dysphoric, effects that may include nausea and vomiting. The frequent user experiences a rush, or warm flushing feeling, in the skin and lower extremities that is often equated with sexual orgasm. This intense euphoria lasts for one to several minutes and is followed by sedation, relaxation, and tranquility lasting up to an hour. This latter period is sometimes called being on the nod. All effects have largely dissipated within 3 to 5 hours, which requires the user to inject at frequent intervals.
Pharmacokinetics plays a very important role in the manner in which opioids are abused. Morphine and many of its derivatives are slowly and erratically absorbed after oral administration, which makes this route suitable for long-term management of pain but not for producing euphoria. In addition, opioids undergo considerable first-pass metabolism, which accounts for their low potency after oral administration. Heroin is more potent than morphine, although its effects arise primarily from metabolism to morphine. The potency difference is attributed to heroin's greater membrane permeability and resultant increased absorption into the brain.
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