Hi. I foudn this information very helpful from fanciers.com:
Some cats vomit all the time; other cats do so relatively rarely. Vomiting is not a sign of the same sort of distress as it is in humans. Because they are carnivores, they need to be able to vomit quickly and almost at will without feeling sick.
On the other hand, a cat that suddenly starts to vomit, or vomits more than usual or in some way demonstrates a departure from its normal habits should be checked by the vet.
Most commonly, a cat vomits because it has hairballs. To check for this, examine the vomit carefully for small grayish pellets or lumps (it doesn't matter what color your cat's hair is). If these are present, then hairballs is the problem. Hairballs occur even with shorthair cats. All cats benefit from regular brushing to help minimize shedding and ingestion of hair. If your cat is vomiting because of hairballs, its normal behavior is not affected. That is, it will be its usual self immediately before and after vomiting.
To help prevent this kind of vomiting, feed your cat on a regular basis some petroleum jelly (aka as Vaseline). If they don't like it, you can try Petromalt, a malt-flavored petroleum jelly. Pats of butter will also work. To give it to them, if they won't eat it of their free will, smear some on top of their paw and they will lick it up as they clean it off. Be careful to rub it in thoroughly, otherwise when they shake their paw, you'll have gobs of vaseline go flying onto the walls or carpet. Give it to them daily for a few days if they've just upchucked or are in the midst of dry heaves; go back down to a weekly dose once they've gotten rid of existing hairballs and this should keep them hairball free. Frequent brushing also helps; every bit of hair on the brush is less hair in your cat's stomach.
Another common reason for vomiting is overeating, particularly dry food. The dry food absorbs water and swells, and then they have to throw it back up. If the vomit looks like a semi-solid tube of partially digested cat food, that's probably what it is.
A cat may vomit when it is allergic to its food. You can check this out by trying another brand of food with substantially different ingredients and no food colorings.
Sometimes cats vomit when they have worms. Consult your vet for a worming appointment.
If the vomit is white or clear, that can be one of the symptoms of panleukopenia, feline distemper. If such vomiting occurs a coule of times over the course of a day or night, a phone call to the vet is in order.
If cats eat something that obstructs their digestive system, they may try to vomit it back up. If you can see some of it in their mouth, DO NOT PULL IT OUT, especially if it is string. You may just cut up their intestines in the attempt. Take the cat to the vet immediately.
If the cat displays other changes of behavior along with the vomiting, you should consult the vet. Eg. listlessness, refusing food along with vomiting may indicate poisoning.
Periodic throwing up can be a sign of an over-active thyroid. This is particularly common in older cats. Your vet can do a blood test and find out the thyroid level. It can also be indicative of a kidney infection: something that your vet can also check out.
In general, as distasteful as it may be, you should examine any vomit for indication of why the cat vomited.
Dietary problems include:
* sudden change in diet
* ingestion of foreign material (garbage, plants, etc)
* eating too rapidly
* intolerance or allergy to specific foods
Problems with drugs include:
* specific reactions to certain drugs
* accidental overdosages
Ingestion of toxins:
* Lead, ethylene glycol, cleaning agents, herbicides, fertilizers, heavy metals all specifically result in vomiting.
* diabetes mellitus
* too little or too much of certain hormones, trace elements, etc.
* renal disease
* hepatic disease
* heat stroke
Disorders of the stomach:
* obstruction (foreign body, disease or trauma)
* assorted gastric disorders
* ulcers, polyps
Disorders of the small intestine:
* intraluminal obstruction
* inflammatory bowel disease
* fungal disease
* intestinal volvulus
* paralytic ileus
Disorders of the large intestine:
* irritable bowel syndrome
* gastrinoma of the pancreas
* peritonitus (any cause including FIP)
* inflammatory liver disease
* bile duct obstruction
* pyometra (infection of the uterus)
* urinary obstruction
* diaphragmatic hernia
* pain, fear, excitement, stress
* motion sickness
* inflammatory lesions
* hiatal hernia
You may now have stains on the carpet that you want to get rid of. Spot Shot, and other stain removers, work well at removing stains. If you're having trouble with bright red or orange stains, you may want to invest in a cat food that doesn't use dyes. That can help considerably in reducing the stain factor."