The aim of the study was a preliminary assessment of the anti-cancer efficacy of the capsaicin-containing habanero pepper extract in dogs. The study was conducted on a group of 50 dogs (33 females, and 17 males aged 6–18 years) diagnosed with different tumours, and 20 dogs (12 females and 8 males, aged 2-12 years) forming a control group. All animals were administered with a diet supplement based on habanero pepper extract containing capsaicin. Observations were conducted for a period of 6 months, during which time the general condition of the animals administered with the extract was monitored, and haematological as well as biochemical examinations were conducted at 2-week intervals in order to assess the tolerance of the animals to the extract. In the animals of the test group, tumour sizes were measured at monthly intervals. After the end of observations, the tumours were removed and subjected to histopathological tests. As a result of habanero pepper extract administration, the tumour size decreased by 5–50% in 15 dogs, the tumours size remained unchanged in 29 dogs, whereas tumour size increased by 10–30% in 5 animals despite the administration of the extract. The extract was well tolerated by the animals. Temporary undesirable symptoms in the form of vomiting or diarrhoea and licking of the anal region, which could stem from its administration, were observed in only nine dogs of the test group and 5 of the control group. An increase in asparagine aminotransferase (AST) activity was observed in 13 dogs of the test group, alanine aminotransferase (ALT) activity was elevated in 11 dogs, whereas alkaline phosphatase (ALP) increased its activity in 18 dogs. Increases in total bilirubin, urea and creatinine concentration were noted in the serum of 10, 9 and 9 dogs respectively. In the control group, the AST activity increased in 7 dogs, ALT in 5, ALP in 5, and total bilirubin concentration in 6. The preliminary clinical observations indicate that the capsaicin-containing habanero pepper extract exhibits favourable effects on different tumours in dogs and is well tolerated by the animals, thus the obtained results are a good sign for future studies on alternative medications used in dog oncology.