Plants of the Allium genus include onions (Allium cepa), garlic (Allium sativum), leeks (Allium porrum) and chives (Allium schoenoprasum). The plants of this genus can irritate the gastrointestinal mucosa of cats and damage their red blood cells.^{1, 2} Damage to the red blood cells leads to anemia.³ These plants are toxic to cats regardless of how they are prepared (whether raw, cooked, processed, dried, etc.). The signs of poisoning – which may not appear until several days later – include vomiting, diarrhea, lethargy, breathing difficulties, rapid heartbeat and pain in the stomach area. The amount that is toxic to a cat is 0.5% of body weight.⁴

Alcohol can cause severe liver and brain damage. Just one tablespoon of alcohol can put a cat into a coma.⁵ The danger comes from the liquid ethanol, which is found not only in alcoholic drinks but also in various foods, including raw bread and pizza dough and rotting fruit. The liquid is quickly absorbed in the gastrointestinal tract and crosses the blood-brain barrier. The clinical signs usually appear within an hour of consuming alcohol. These include depression of the central nervous system, lack of coordination (ataxia), extreme tiredness (lethargy), abnormally low body temperature (hypothermia) and the build-up of acid in the blood and body (metabolic acidosis). A cat suffering from alcohol poisoning can also go into a coma and suffer respiratory depression (shallow breathing).⁶

Caffeine is a stimulant found in coffee, black and green tea, energy drinks and various soft drinks. It is also present in chocolate (especially dark chocolate). After consuming caffeine, cats may experience restlessness and excessive thirst. They may also pee more, breathe faster, or suffer from palpitations or convulsions. The cat may vomit (which is actually beneficial as it will remove some of the toxic caffeine from the body) and suffer from diarrhea. In the most extreme cases, a cat may go into cardiac and respiratory arrest.^{7, 8} The amount of caffeine that is toxic to cats is 80 to 150 mg per kilogram of body weight.⁹

Chocolate contains both caffeine (see above) and theobromine. Theobromine has a similar effect to caffeine but is somewhat weaker. The greater the cocoa content in the chocolate, the higher the proportion of theobromine in it. For this reason, dark chocolate is more dangerous to cats than white or milk chocolate. Symptoms of poisoning occur in a cat at 200 mg per kilogram of body weight.^{10, 11} Eating chocolate can lead to cardiac arrhythmia, muscle tremors or seizures. Other symptoms include vomiting, diarrhea, flatulence, panting and increased drinking.¹² 

Citrus fruits such as oranges, limes and lemons contain varying amounts of citric acid and essential oils that can cause irritation in cats. They can also lead to depression of the central nervous system (decrease in neurological functions) if taken in large amounts. Small doses, such as from eating a piece of fruit, can result in stomach upset, vomiting and diarrhea.^{13, 14, 15} 

These foods contain active substances that target the kidneys.¹⁶ After eating grapes, raisins or sultanas, a cat may suffer from kidney damage and even failure.¹⁷ That said, reports of such poisoning in cats are relatively rare (this may be because cats instinctively avoid eating these particular fruits in the first place). Nevertheless, these foods should always be kept out of reach of cats.

The effects of milk and dairy products will depend on whether a particular cat is lactose intolerant, the signs of which include diarrhea, vomiting, flatulence and abdominal pain.¹⁸ It is generally adult cats that are lactose intolerant. This is because once kittens have been weaned, they no longer produce an enzyme called lactase, which breaks down the lactose in their mother’s milk. (Read Milk is not good for cats).

Nuts are harmful to cats in many ways. They contain lots of fat, which – depending on the quantity eaten – can cause pancreatitis.¹⁹ In addition, their high fiber content and the fact that they are difficult to digest can lead to stomach upset. Nuts can also be a choking hazard for cats or cause intestinal blockages.²⁰

Depending on the species, wild mushrooms may contain various toxins and can be very dangerous to cats. If a cat eats a toxic mushroom, they can very quickly fall ill and even die. General symptoms associated with mushroom poisoning include vomiting, diarrhea and excessive thirst and peeing. The cat may walk unsteadily or lurch, and they may appear to be hallucinating. Kidney failure may also occur.^{21, 22}

Raw eggs should not be fed to cats for two reasons. First, they carry a risk of salmonella. Second, the egg white contains the protein avidin. Eating substantial quantities of raw eggs leads to a reduction or inhibition of the absorption of biotin (vitamin B7) in the gastrointestinal tract.^{23, 24, 25} This is serious because cats need biotin for a number of reasons. The vitamin not only contributes to a healthy coat and skin and a well-functioning metabolism²⁶ but also supports the thyroid and adrenal glands, among other things.²⁷ Visible signs that a cat may be suffering from biotin deficiency include dandruff, itching and eczema.²⁸

Feeding raw fish to cats can have negative effects on their health. Raw fish and the guts of certain fish contain enzymes called thiaminases. These split up vitamin B1 (thiamine), making it inactive.²⁹ However, thiamine is very important to the cat’s metabolism. If a cat has thiamine deficiency, they cannot fully utilize the energy contained in their food. As a result, they suffer from a loss of appetite, followed by symptoms such as weight loss and weakness.³⁰ Cats affected by thiamine deficiency show signs of neurological disorders of varying severity. These include lack of coordination (ataxia) and convulsions. Sometimes cats also adopt an unusual posture, either tilting their head towards their chest or arching their entire body as if in a seizure (opisthotonos). If nothing is done about the condition, a racing heart and arrhythmia may develop. The affected cat may go into a coma and eventually die from vitamin deficiency.

Veterinarians advise caution when feeding raw (poultry) meat as it can be contaminated with bacteria such as Campylobacter.^{31, 32} Campylobacter infection usually affects younger animals (less than six months old) or stressed and weakened animals.^{33, 34} The main symptom of infection with these bacteria is diarrhea due to gastrointestinal inflammation. Other signs are abdominal pain, occasional vomiting and reduced appetite.³⁵ Infection with salmonella can also be accompanied by diarrhea and (sometimes) vomiting.

Both infections are zoonotic, which means they can be passed on from animals to humans.³⁶

The yeast in raw dough produces carbon dioxide and alcohol from sugar and starch. This normal process is dangerous when it takes place inside a cat’s stomach. Unfortunately, the warm and moist environment of the stomach provides ideal conditions for the yeast dough to rise, producing both carbon dioxide and alcohol. As a result, the stomach expands (gastric dilatation), and the animal’s abdomen becomes painfully distended. If gastric torsion (twisting of the stomach) occurs, the inlet and outlet of the stomach can become closed. This can be life-threatening for the cat, as the further expanding stomach presses against other organs and prevents them from working properly (for example, if pressure is exerted on the diaphragm, breathing becomes difficult³⁷). If left untreated, gastric torsion leads to death. Alcohol intoxication may also occur, as shown by symptoms such as disorientation, physical weakness, uncoordinated movements and possibly seizures (see also “Alcohol” above).^{38, 39} 

Liver contains vitamins A and B, among others. Eating small amounts is not harmful. However, if a cat is fed raw liver (or other foods rich in vitamin A) for several weeks to months, vitamin A poisoning will occur. When cats ingest very large amounts of vitamin A, they can develop symptoms such as vomiting, drowsiness and irritability, as well as peeling of the skin. An overdose of vitamin A over a long period leads to symptoms including a poor coat, rough or dry skin, weakness, weight loss, constipation, painful or restricted movements and excessive bone development.^{40, 41}

Tuna that is suitable for human consumption can cause digestive upset in cats, even if it is only occasionally offered as a treat.⁴² If it is fed regularly and in large quantities to cats, steatitis (yellow fat disease) may develop. This disease, which is quite rare in cats, is characterized by a painful inflammation of the fatty tissue, accompanied by the appearance of firm, painful lumps under the skin. Treatment includes pain-relieving therapy and, if necessary, surgery to remove the affected tissue.^{43, 44}

Whether – and to what extent – this sugar substitute is dangerous to cats has not been sufficiently investigated yet. A small study was carried out in 2018 aiming to find out whether xylitol is as dangerous to cats as it is to dogs. The results suggested that xylitol has no toxic effect on cats.⁴⁵ These findings are supported by the Justus Liebig University, which also states in its article “Xylitolvergiftung beim Hund” (“Xylitol Poisoning in Dogs”) that xylitol is not toxic for cats.⁴⁶