Dogs may vomit for a wide range of reasons. It may be an isolated episode, for example due to eating too large a meal or indigestible foods, or vomiting may be a symptom of a more serious condition, such as poisoning or the presence of a foreign body.
It is very important for dog owners to distinguish when to be concerned and when to remain calm.
If the dog vomits only once or twice in succession, then immediately goes back to playing or asking for another portion of food, there is probably no need to worry.
If, on the other hand, vomiting persists for a long time and other symptoms also appear, such as prolonged diarrhea, fever, lethargy and loss of appetite, especially for several days in a row, you should consult your veterinarian. In particular, some signs may be warning signals of very serious conditions, such as traces of blood in the vomit or feces, or greenish-colored vomit.
What are the causes of vomiting in dogs?
If a dog vomits, there may be many different causes. In most cases it is a sporadic event that ends within a few minutes or, at most, after a few hours.
However, vomiting may also occur in the case of certain conditions and diseases. 
The most common situations that may promote vomiting in dogs include:
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Eating the meal too quickly;
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Portions that are too large;
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Sudden change of food;
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Food intolerances or allergies;
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Ingestion of objects, bones or spoiled food;
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Motion sickness;
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Heatstroke;
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Ingestion of grass or plants;
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Antibiotic or chemotherapy treatments that may trigger vomiting.
If vomiting recurs with some frequency and tends to occur for several days in a row, you should consult your veterinarian. In these cases, various conditions and diseases may underlie this symptom, requiring a specific diagnosis and treatment.
Less common causes include mainly:
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Gastrointestinal infections caused by viruses or bacteria, often accompanied by diarrhea and fever. A particular case is Parvovirus, the virus responsible for viral gastroenteritis in dogs. It causes intense vomiting and prolonged diarrhea, with traces of blood in the feces. This is a very dangerous and potentially fatal infection, especially for puppies;
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Presence of foreign bodies in the stomach or intestine, such as bone fragments, toys and other objects ingested by the animal;
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Parasites: these are found in the dog’s intestine, where they cause so-called “worm infestations”, mainly characterized by diarrhea, but they may also cause vomiting if the infestation is severe;
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Prolonged constipation: if the dog does not pass feces regularly, ingested food can no longer pass through the blocked intestine and may therefore come back up through vomiting;
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Gastritis: inflammation of the inner lining of the stomach. Mild forms are fairly common in dogs and are accompanied by temporary vomiting and abdominal pain;
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Stomach ulcers: actual wounds located in the stomach. The causes may vary, but in most cases they are a complication of prolonged gastritis;
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Diabetes: diabetic dogs may experience an increase in “ketones”, acidic substances that accumulate in the blood. In this case the animal will attempt to expel some of these acids through vomiting, thereby reducing blood acidity;
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Pancreatitis, inflammation of the pancreas;
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Peritonitis: inflammation of the membrane lining the inner abdominal wall. It is almost always accompanied by high fever and abdominal pain;
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Poisoning: a dog that has ingested poison shows a series of often unmistakable symptoms, such as increased salivation, intense tremors, vomiting, diarrhea and inability to stand. In these cases, contact your veterinarian immediately or take the dog to the nearest clinic;
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Kidney failure: this occurs when the kidneys can no longer filter the blood, so substances normally eliminated in the urine accumulate in circulation. Vomiting is always accompanied by a marked increase in thirst and loss of appetite;
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Liver disease: in this case vomiting may take on a greenish tinge, due to traces of bile, the substance produced by the liver;
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Brain tumor;
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Stomach or intestinal cancer.
What to do if your dog vomits?
If your dog vomits occasionally, without showing other symptoms, try to identify the possible cause by monitoring all of its activities.
You may discover that several toys are missing, or that houseplants have been broken and chewed. In these cases, simply remove anything that might tempt your dog.
If, on the other hand, vomiting is particularly intense and repeated, go to your veterinarian immediately, who can tell you what to do based on the precise diagnosis.
As a general recommendation in case of vomiting, for the two hours following the onset of vomiting, do not give your dog either food or water. If the stomach is very irritated, anything that causes it to expand may further stimulate vomiting.
It is nevertheless important to prevent the dog from becoming dehydrated, especially in the presence of profuse diarrhea. Therefore wait at least one hour, then begin offering your dog small amounts of water at intervals.
If no further episodes of vomiting occur within twelve hours, you can start feeding the dog. Choose light, digestible foods such as chicken or boiled rice, naturally without seasoning.
A good idea may be to offer rusks or dry bread, which can absorb the excess acidity typical of vomiting.
It is not true that…
If the dog vomits, you should make it eat grass.
Usually, when nauseated, the dog itself seeks out and eats grass in the garden or countryside, then vomits immediately afterward. Grass is in fact an irritant to the stomach that stimulates vomiting almost immediately. If the stomach is already irritated, there is no reason to irritate it further.
If poisoning is suspected, a bowl of milk will get the dog back on its feet.
This belief is both true and false at the same time. Milk is an excellent antidote only for certain substances, while for others it is completely useless. Therefore, do not attempt to improvise remedies for intoxication or poisoning. Instead, take your dog to the veterinarian immediately if you suspect it has ingested something toxic.
If the dog has vomited, we should check our medicine cabinet.
Always remember that a dog is not a human being. Therefore, medications we use for our illnesses are not always suitable for our friend as well. Before giving it common antacids or antiemetics, always consult your veterinarian.