If you want to know what withdrawal is like, read this scene disturbing scene in Seth Morgan's novel, Homeboy, in which his incarcerated hero suffers extreme bowel activity after months of heroin-provoked constipation:

"Then the Big Hurt pushed aside all thinking and Joe could only lie hugging his cramped middle and suffer the agony that gnawed on itself, metastasised, grew like a cold malignant fetus in him. A reeking viscous sweat like cold bacon drippings filled him. The jailhouse stinks... dizzied him with nausea.

"Orgasm after electric hairtrigger orgasm convulsed his groin. His entire being became the shortcircuiting terminus of a billion scraped and shrieking nerves.

"And then came ripping down from his intestines that glacial fecal boulder compacted by months of bowel paralysis, and through gritted teeth he cried: 'Christ! The Yenshee baby.'

"He bailed out of his bunk and staggered to a rear toilet where he sat bent double for minutes or hours, he didn't know, trying to pass this bowel monster; until sudden pain flashed the darkness and he felt himself tearing in two. Blood vomited into the toilet. His sweatslick buttocks slipped off and he was on the floor, shrieks percussing his skull; and from a great distance heard Smoothbore shouting at the bars: 'MAN DOWN!' "

Not nice.