Bulk forming agents

Psyllium (Metamucil)
Calcium polycarbophil
Methylcellulose

Bulking agents add bulk to the stool. The increased bulk stimulates the natural contractions of the intestine. These agents act gently and are the safest way to promote regular bowel movements. But the main drawback is this agents work slow. These agents are taken in small amounts at first and the dose increased gradually until regularity is achieved. One should drink plenty of fluids for the bulking agents to work.

Stool softeners

Docusate sodium (Colace, DOSS)

Stool softeners increase the amount of water that the stool can hold. These laxatives work like detergents that decrease the surface tension of the stool, allowing more water to penetrate the stool more easily and soften it. The increased bulk stimulates the natural contractions of the large intestine and helps the softened stools to move more easily out of the body

Mineral Oil
Mineral oil softens the stool and facilitates its passage out of the body. But mineral oil may decrease the absorption of certain fat-soluble vitamins.

Stimulants

Senna (Senekot)
Cascara 
Bisacodyl (Ducolax)
Phenolphthalein
Castor Oil

Stimulants directly stimulate the walls of the large intestine, causing it to contract and move the stool. These laxatives generally work in 6-8 hours but often cause cramping. In suppositories form, these laxatives often work in 15-60 minutes. Prolonged use of stimulants can damage the large intestine.

Saline Laxatives

Milk of Magnesia
Magnesium citrate
Phosphate sodium

Osmotic agents pull large amount of water into the large intestine, making the stool soft and loose. The excess fluid also stretches the walls of the large intestine, stimulating contractions. These agents consist of salts or sugars that are poorly absorbed. These agents generally work within 3 hours. These laxatives are best to treat constipation than to prevent it.

 

Reference: Merck Manual Home Edition 1999