Laxatives are drugs that increase water in the intestine, promote peristalsis, soften stool or lubricate the intestine to facilitate defecation. They are mainly used clinically for functional constipation. There are three types of laxatives: volumetric, stimulant and lubricating laxatives. That we are in the daily how to scientifically understand and choose the right laxative a, volumetric laxatives 1, non-absorbable salts magnesium sulfate and sodium sulfate also known as salt laxatives. In the intestinal tract is difficult to absorb, a large number of oral formation of hypertonic pressure and prevent the absorption of water in the intestine, expand the intestine, stimulate the intestinal wall, promote intestinal peristalsis. In addition, magnesium salt can also cause the duodenum to secrete cholecystokinin, this hormone can stimulate intestinal fluid secretion and peristalsis. General fasting application, and drink a lot of water, 1 to 3 hours that the diarrheal effect, the discharge of liquid stool. The diarrheal effect is very strong, so the clinical mainly used to exclude intestinal toxins and some intestinal worm medicine after taking the worm with the drug. Oral high concentration of magnesium sulfate or direct injection into the duodenum with a catheter, due to reflexive cause relaxation of the common bile duct sphincter, gallbladder contraction, biliary effect. Can be used for obstructive jaundice, chronic swollen cystitis. Magnesium sulfate, sodium sulfate diarrhea effect is more dramatic, can cause reflex pelvic congestion and water loss. Menstruation, pregnant women and the elderly use with caution. 2, food fiber and other substances (1) lactulose is a disaccharide of galactose and fructose. It is not digested and absorbed in the small intestine, so it can induce diarrhea. The unabsorbed part enters the colon and is metabolized by bacteria into lactic acid, etc., which further increases the intestinal osmotic pressure and causes light diarrhea. Lactulose can also lower the pH of the colonic contents and reduce the formation of ammonia in the intestine; H+ can also form ammonium ions with the generated ammonia and not be absorbed, thus reducing blood ammonia. It can be used for chronic portal hypertension and hepatic encephalopathy. It should be noted that loss of water and electrolytes due to diarrhea can worsen hepatic encephalopathy. (2) Food cellulose including natural and semi-synthetic polysaccharides and cellulose derivatives such as methyl cellulose and carboxymethyl cellulose in vegetables and fruits are not absorbed by the intestine, increasing intestinal volume and keeping stool wet and soft, with good laxative effect. Can prevent and control functional constipation. Second, contact laxatives old name “stimulant laxatives”. Can affect intestinal activity and absorption of water and electrolytes in the intestinal mucosa and cause diarrhea of a class of drugs. Including anthraquinone and diphenylmethane, such as rhubarb, senna and aloe and other plant laxatives contain anthraquinone glycosides. It mainly acts on the large intestine and has no effect on the absorption function of the small intestine, so it can be used for acute and chronic constipation. Phenol peptide is also a contact laxative. 1, phenolphthalein after oral administration in the intestine and alkaline intestinal fluid meet to form soluble sodium salt, can promote colon peristalsis. It is mild and suitable for chronic constipation. About 15% of oral phenolphthalein is absorbed. It is excreted in the urine, which is red if the urine is alkaline. It is partially excreted by bile and has hepatic and intestinal circulation to prolong its action, so the effect of one dose can be maintained for 3-4 days. In case of anaphylactic reaction, enteritis, dermatitis and bleeding tendency occur. The same kind of drugs pisacodyl is used for constipation or X-ray, endoscopy or pre-operative evacuation of intestinal content. 2, anthraquinones rhubarb, senna and aloe vera and other plants, containing anthraquinone glycosides, orally decomposed by bacteria in the large intestine into anthraquinone, can increase the colon propulsive peristalsis. 6-8 hours after the drug defecation, commonly used in acute and chronic constipation. Third, lubricating laxatives lubricating laxatives is through the local slippery and soften the stool and play a role. Suitable for the elderly and hemorrhoids, anal surgery patients. 1, liquid paraffin for mineral oil, not digested and absorbed by the intestine, produce the effect of slippery intestinal wall and soften the stool, so that the stool can be easily excreted. 2.Glycerin Injected into the anus with 50% concentration of liquid, it stimulates the intestinal wall to cause defecation response due to high osmotic pressure and has local lubrication effect, causing defecation within a few minutes. It can be applied to children and the elderly.