A common cause of poisoning in dogs is ethanol, a colourless liquid that is also known as alcohol. Alcoholic drinks and food (including rotting fruit, which ferments to produce alcohol) can cause nausea, vomiting, diarrhoea, lack of coordination, disorientation, walking unsteadily, excessive thirst and peeing, and other symptoms. More serious signs of alcohol poisoning include slowed breathing, tremors, seizures, coma and even, in the very worst cases, death.^{1, 2}

Avocados are less dangerous to dogs than they are to some other animal species (rabbits, guinea pigs, rats, budgies, cockatiels and canaries can die from eating avocados). The main risk to dogs lies in the high fat content of the avocado, which can cause mucosal irritation in the gastrointestinal tract and pancreatitis (inflammation of the pancreas). Another danger comes from the large pit of the fruit: if swallowed, it can get stuck in the digestive tract and may have to be surgically removed.^{3, 4, 5}

Though it may be hard to believe, bones can be harmful to your dog’s health and welfare. For example, a dog’s teeth can break on a hard bone; their tongue, cheek and soft palate can be pierced by sharp splinters. Pieces of bone that are accidentally swallowed can sometimes become lodged in the oesophagus or in the trachea (windpipe). If sharp bone fragments get into the gastrointestinal tract, they can damage the mucous membrane. And in extreme cases, the gastrointestinal wall itself can be pierced, causing food and intestinal contents to enter the abdominal cavity. This can result in peritonitis, which can be fatal. ^{6, 7}

Caffeine is a stimulating ingredient in various stimulants, such as coffee, black and green tea, cocoa, energy drinks, cola and mate and chocolates (especially dark). Visible signs of caffeine poisoning in a dog are its restless behaviour. The animal simply does not come to rest, it is jittery and may constantly run back and forth. Caffeine increases blood pressure and heart rate; dangerous cardiac arrhythmias may occur. The affected animal may lose control of the muscles, tremble and have seizures. Since caffeine affects the gastrointestinal tract, diarrhoea and vomiting cannot be ruled out consequently. Regardless of age or size, caffeine can damage major organs such as the liver, heart, kidneys, lungs and central nervous system. If left untreated, a large amount of caffeine consumed will lead to death. ^{8, 9}

The problematic thing about chocolate is the stimulants it contains, which can be dangerous for dogs – especially small ones – in high doses, such as the strong substance theobromine. Theobromine is the main toxic substance and very similar to caffeine. Both substances are used in medicine as heart stimulants and to dilate the blood vessels, among other things.¹⁰ Dark chocolates, such as those with 90% cocoa content) have a significantly higher quantity of theobromine than white or milk chocolate. Theobromine stimulates the central nervous system and the heart, increases blood pressure and causes indigestion.¹¹ Signs of chocolate poisoning include agitation, nervousness, balance disorders, palpitations and epileptic seizures. Severe poisoning can cause loss of coordination and seizures and can even lead to coma and death.^{12, 13}

The stems, leaves, peel, fruit and seeds of citrus plants contain varying amounts of citric acid and essential oils that can cause irritation and possibly even depression of the dog’s central nervous system if eaten in large quantities. Smaller doses – such as eating the occasional piece of citrus fruit – should not cause major problems, although it might lead to a mild stomach upset.^{14, 15}

The risk posed by raw eggs is that of salmonella poisoning.¹⁶ Eggs contaminated with salmonella can cause the infectious disease salmonellosis in dogs. Affected dogs suffer from fever, vomiting, diarrhoea and loss of appetite. Depending on the severity of the infection, dehydration, gastrointestinal inflammation and shock may also occur. In extreme circumstances, pregnant bitches may suffer a miscarriage.¹⁷

Fatty foods such as drippings can cause pancreatitis in dogs.¹⁸ The most common clinical signs of pancreatitis include nausea, vomiting, fever, lethargy, abdominal pain, diarrhoea and decreased appetite. In extreme cases, it can lead to acute shock or even death.¹⁹

Dogs can get salmon poisoning if they eat raw or insufficiently cooked fish. However, the term 'poisoning' is somewhat misleading as the disease is in fact a bacterial infection. Raw or semi-cooked salmon, trout, rainbow trout or other anadromous (swimming upstream) fish can sometimes be infected with a parasite (Nanophyetus salmincola). This flatworm is not in itself harmful to dogs, but it can be infected with a specific bacterium (Neorickettsia helminthoeca). After being swallowed, the flatworm settles in the dog’s intestine, which enables the bacteria to enter the bloodstream and travel to various organs, where they cause infection. Symptoms usually appear within a week of eating the infected fish. Common signs are loss of appetite, vomiting, diarrhoea (which may also contain blood), fever, weakness, swollen lymph nodes, weight loss, discharge from the eyes or nose, faster breathing, increased heart rate, muscle tremors and seizures. If the infection is not treated, most dogs die within two weeks.^{20, 21, 22}

The seeds, pips and stones of these fruits are the main problem here. Persimmon seeds, for example, can cause inflammation of the small intestine or intestinal obstruction.²³ There is a risk of tooth fractures if a dog bites on hard pits. Some fruits (e.g. peaches) have sharp-edged pits that can injure the oesophagus when the dog tries to swallow them. Pits can sometimes get stuck in the oesophagus. Blockages can also occur if the dog swallows a large pit or many small ones, which get stuck in the intestine. Blockages of this kind often require surgery.

Pits and seeds of common fruits such as apricots, peaches and apples may have substantial amounts of chemicals that are metabolised to produce the poison cyanide. ^{24, 25, 26}

Onions, chives and leeks – which belong to the Allium genus – are all harmful to dogs, but garlic is the most dangerous. These plants and herbs are harmful to dogs regardless of how they are prepared – whether dried, cooked or powdered. Vomiting – the first sign of poisoning – can occur immediately after eating. However, further clinical signs may appear days later: inflammation of the gastrointestinal tract, irritation of the mouth, drooling, nausea, abdominal pain, vomiting and diarrhoea. The potentially lethal ingredient of these Allium plants damages the red blood cells, so they become brittle and burst. With the dwindling number of red blood cells, the vital blood supply to the organs is jeopardised, and in the worst cases this can lead to death. The first visible signs of this are faster breathing, breathlessness, physical weakness, discoloured urine and pale gums.^{27, 28, 29, 30}

It is the tartaric acid found in grapes, raisins and sultanas that seems to trigger symptoms of poisoning.³¹ The fruits are considered highly toxic to dogs – even in small amounts. Early symptoms of grape or raisin poisoning are vomiting, diarrhoea, loss of appetite and lethargy. More serious signs don’t appear until 24 to 48 hours later. By then, kidney damage has already occurred. Symptoms of kidney failure include nausea, drinking more and peeing more. If the dog is not treated in time, they will fall into a coma, and it is very unlikely that they will recover.^{32, 33}

Feeding raw or half-cooked meat to a dog can be dangerous, as it is impossible to know for certain if the meat is contaminated. Raw meat products can contain high levels of bacteria such as salmonella, clostridium perfringens and campylobacter.³⁴ Infections with these bacteria can cause intestinal inflammation³⁵ accompanied by diarrhoea, vomiting and fever.³⁶ There can also be gastrointestinal inflammation with the accompanying symptoms – see 'Eggs (raw)'.

Special care should be taken with raw pork. The virus sometimes found in this meat (a herpes virus) can trigger the fatal Aujeszky’s disease, also known as pseudorabies. The first signs that a dog may have the disease are restlessness and aggressive behaviour. As the disease progresses, there may be severe salivation, vomiting, diarrhoea and, in particular, severe itching, which may lead to self-injury through gnawing or scratching. In the final stage, neurological disorders such as convulsions and paralysis occur. Sadly, in the majority of cases, the affected dog will die within 48 hours of the first symptoms appearing.^{37, 38}

These products are only harmful to dogs that are allergic³⁹ to them or are lactose intolerant. Affected dogs suffer from itching and skin reactions such as pustules (food allergy), or they react with flatulence, diarrhoea and vomiting (lactose intolerance).

Depending on the fat content, dairy products can also cause pancreatitis (see 'Fatty foods').

It is well known that mushrooms growing in the wild can be extremely dangerous or even deadly to dogs. But what about the mushrooms you buy at the supermarket? Basically, it is thought that all edible, cultivated mushrooms that are suitable for humans are also safe for dogs.^{40, 41} However, while mushrooms are a source of B vitamins and minerals, they are not a necessary part of a dog’s diet. So, it may be better to remove them from your dog’s diet altogether. Safety first!

Macadamia nuts must never be fed to a dog. These oil-rich nuts can poison our canine companions, although the exact mechanism of toxicity has not been identified yet. The signs of macadamia poisoning appear in several phases. The first symptoms – which include vomiting, fever and lethargy – appear 3 to 6 hours after eating a dangerous amount. In the next phase (6 to 12 hours after eating), the dog will walk unsteadily and seem unwilling to use their hind legs. They may be unable to stand and suffer from joint stiffness or muscle tremors.

Nuts in general have a high fat content (carrying the risk of pancreatitis) and are extremely calorific. For this reason, even the types of nuts that are often seen as harmless should have no place in a dog’s diet.^{42, 43, 44, 45}

Eating too much salt can lead to excessive thirst and peeing, and it can sometimes result in sodium ion poisoning. Symptoms of excessive salt consumption can include vomiting, diarrhoea, tremors and seizures. In extreme cases, the animal may die.^{46, 47}

Sugary foods can lead to obesity, dental problems and diabetes.⁴⁸

Tomatoes belong to the nightshade family.⁴⁹ They contain a substance called solanine, which can be toxic to dogs in large quantities.⁵⁰ Most of the solanine in tomato plants is found in the green parts, such as the leaves, stems and stalks. Green and unripe tomatoes contain more solanine than ripe ones. However, since even ripe tomatoes contain some solanine, there is always a small risk of poisoning with every tomato your dog eats.⁵¹

Xylitol is a naturally occurring substance found in berries, plums, corn, oats, lettuce and cauliflower. It is often used in human foods as a sugar substitute. Xylitol causes a strong insulin release in dogs, which results in a drastic drop in blood sugar (hypoglycaemia).^{52, 53} If left untreated, hypoglycaemia can be life-threatening. Symptoms include tremors, apathy, weakness, lack of coordination, disorientation and impaired vision. Epileptic seizures, liver failure and coma are also possible. Even small amounts of xylitol can be dangerous to dogs.^{54, 55}

If eaten, raw dough expands in the warm and moist environment of a dog’s stomach. This leads to bloating and, in the worst cases, to a dangerous condition known as gastric torsion (twisting of the stomach).^{56, 57} The bloated stomach presses against the diaphragm, making it difficult to breathe. The yeast and sugar found in raw dough also ferment in the stomach, and this produces alcohol.⁵⁸ If the alcohol enters the bloodstream, it causes symptoms including nausea and vomiting, as well as more serious or even life-threatening conditions (see 'Alcohol'). Caution is also advised with raw pizza dough.