The major types of laxatives can be divided into several categories. Bulking agents, also known as fiber supplements or bulk-forming laxatives, are generally considered to be the safest laxatives available. They work slowly and gently, and are taken with water. Bulking agents are not digested, but instead absorb water in the intestine and then swell to soften stool. The water they add to the stool increases its bulk, which stimulates the natural contractions of the large intestine. Patients take small amounts of the medication initially, then gradually take larger doses until they achieve regularity. A drawback of these laxatives is that they may prevent proper absorption of some medications.

As their name implies, stimulant laxatives contain stimulants, including senna, cascara and bisacodyl, which trigger rhythmic muscle contractions in the intestines. When taken orally, they will usually cause a bowel movement within six to eight hours. In suppository form, they can produce a bowel movement in as little as 15 minutes to an hour. Stimulants are considered to be the harshest laxatives and should only be used every three days under close physician supervision. Some studies have found a link between increased cancer risk and phenolphthalein, an ingredient found in some stimulant laxatives. The U.S. Food and Drug Administration (FDA) has proposed a ban on all over-the-counter products containing phenolphthalein, and most manufacturers of laxatives have either replaced the substance or are planning to remove it from their products.

Osmotic agents draw water from surrounding tissues into the bowel, which creates a soft stool mass and stretches the walls of the large intestine, thereby stimulating contractions. There are three types of osmotics:

• Saline laxatives. Used to rapidly empty the lower intestine and bowel, but are not recommended for long-term constipation or repeated use. They have more limited uses than other laxatives and are usually used to provide rapid relief. For example, they may be used in order to provide a stool sample, and to eliminate foods or drugs rapidly from the body in cases of poisoning or overdose. In smaller doses, they may also be used as antacids.
  
  
• Lactulose laxatives. Have a sugar-like composition and work like saline laxatives. However, they produce results more slowly and are used more often in the long-term treatment of chronic constipation. They are also sometimes used to reduce ammonia levels in the blood. This type of laxative is available by prescription only.
  
  
• Polymer laxatives. These cause water to be retained in stool, softening the stool and increasing the number of bowel movements. Polymer laxatives are used on a short-term basis to treat constipation.

Other common types of laxatives include:

• Stool softeners. Add moisture to the stool and prevent dehydration, which keeps the stool from becoming hard and dry. This effect is achieved by decreasing the surface tension of the stool, which allows more water to penetrate the surface. Physicians often recommend these laxatives following childbirth or surgery. Stool softeners do not actually trigger bowel movements, but instead allow the patient to have a bowel movement without straining.
  
  
• Lubricants. Taken orally or in suppository form, lubricants provide a waterproof coating to the stool that locks moisture into the stool and allows it to move through the intestine more easily.
  
  
• Carbon dioxide-releasing agents. These laxatives are available as suppositories and once inserted form the gas carbon dioxide, which then pushes against the intestinal wall and triggers contractions that move along a stool mass.

• Chloride channel activators. Chloride channels are present in the intestinal epihelial cells and when stimualted, there is an increase in fluid secretion in the small intestine. This, in turn, permits the passage of stool. Chloride channel activators are used in treatment of chronic constipation. Lubiprostone is a selective chloride channel activator and is a newly approved medication for treatment for chronic constipation.

Examples of some of these laxatives include:

In addition, some products combine laxatives. These products may be more likely to cause side effects than single-action laxatives. Combination laxatives include:

• Bulking agent/stimulant
• Bulking agent/stimulant/stool softener
• Osmotic/lubricant
• Stimulant/stool softener