Cancer is the second leading cause of death worldwide after cardiovascular diseases, and its incidence is growing. The efficacy of cancer treatment is increasing due to a better understanding of its biology and improvements in diagnostic and therapeutic methods. Active participation by patients in the diagnostic and therapeutic process may increase their compliance and well-being. However, greater patient awareness, more accessible public data, and determination often prompt them to seek unproven alternative therapies.

Complementary and alternative medicine (CAM), as opposed to evidence-based medicine (EBM), is not grounded in well-designed clinical studies, and thus may not be effective or may even harm patients. Complementary medicine is used in addition to standard medicine, whereas alternative medicine is used in lieu of standard methods.

Patients diagnosed with cancer are frequently confused due to the unpredictability of the situation, stress, and fear of the future of themselves and their families. The willingness to actively participate in the therapeutic process may prompt them to seek allegedly effective CAM options. Patients attempt these methods to increase treatment efficacy, alleviate treatment side effects, or improve their physical and mental condition. However, in many instances, patients replace main treatments with alternative methods, which may considerably worsen their prognosis.

The use of CAM in cancer patients has been consistently increasing [1]. For example, in a nationwide survey carried out in the Nepal, 32% of cancer patients reported using alternative therapies [2]. In another study of almost 1500 cancer survivors, 67% reported ever using CAM, and 43% had used CAM in the past year [3]. Alternative therapies are not subject to any formal regulations in Poland, and no public education programs address this issue. Consequently, patients often rely on knowledge from the Internet, which is frequently untrustworthy. The growing popularity and heterogeneity of CAM methods make them an important issue for patient–doctor relations in Poland and other Central European countries [4]. A recent study from Poland demonstrated that an astonishing number of CAM practices offered to manage multiple entities [5].

One of the reasons for seeking unconventional methods is the lack of time and understanding of medical staff. Cancer therapy requires a complete understanding of both parties and a truthful dialogue to ensure the safety and well-being of the patient. In addition, a sincere relationship with the treating physicians and their basic knowledge of alternative treatments may significantly influence patients’ decision-making process.

The increasing use of CAM by cancer patients constitutes a challenge for health care systems. Apart from social education, good communication between cancer patients and medical staff is crucial in managing this problem. This aim may be achieved by competence, understanding, patience, and adequate support for patients.

Health care professionals generally question the value of CAM and see no need to increase their expertise on this subject. However, having a basic knowledge of CAM may facilitate discussion with patients and influence their decisions.

2. Chlorella

Chlorella is a unicellular alga from the class of green algae that is increasingly being added to yogurts, juices, and smoothies in powder form. It is rich in protein; vitamins (particularly B vitamins); trace elements such as magnesium, potassium, iron, calcium, and zinc; fiber; and omega-3 fatty acids. The antioxidant and immunomodulatory properties of chlorella result from increasing the activity of NK cells and stimulating the production of interferon-γ, interleukin-12, and interleukin-1β. Ref. ^[6][62] Hot water extract of Chlorella vulgaris induces apoptosis and DNA damage in non-small-cell lung cancer cell lines ^[7][63]. Animal and in vitro studies have shown its antiproliferative effects on liver and colorectal cancer cells ^[8][9][6,7]. Lycopene isolated from chlorella inhibited the growth of prostate cancer cells in ^[10][64]. In an animal model, chlorella extract reduced bone marrow suppression caused by cisplatin ^[11][8]. Clinical data on chlorella treatment include only a small group of breast cancer patients ^[12][31]. According to a survey, chlorella extract decreased the severity of chronic weakness and dry skin in this group. The anticancer activity of chlorella has not been the subject of clinical trials.

3. Beet Juice, Carrot Juice

Many studies confirm the role of diet, mainly in cancer prevention. A diet including high amounts of lutein-rich vegetables such as spinach, broccoli, lettuce, tomatoes, oranges, carrots, and celery has been proven to reduce the risk of developing proliferative diseases. The inclusion of these foods in the diet can reduce the risk of colorectal cancer ^[13][65]. A citrus-rich diet reduces the risk of laryngeal cancer, and a diet high in fruits and vegetables reduces the risk of pancreatic cancer ^[13][14][65,66]. Consumption of carotenoid-rich foods inhibits DNA damage, and the betanin in beet juice induces apoptosis in breast cancer cells ^[15][16][67,68]. In vitro studies have shown that beet juice may increase the anticancer effect of doxorubicin ^[17][9]. A case report suggested that the combination of chlorambucil with beet juice and carrot juice is beneficial in a B-CLL leukemia patient ^[18][32]. Finally, the consumption of large amounts of carrot juice and beet juice was shown to reduce the anticancer effects of cisplatin ^[19][33].

4. Hemp

Hemp and hemp-derived cannabinoids (i.e., substances that act on cannabinoid receptors) are available for medical treatment in many countries. Individual preparations differ in delta-9-tetrahydrocannabinol and cannabidiol (CBD) content. In Poland, an aerosol preparation containing THC and CBD is registered to treat spasticity symptoms in patients with multiple sclerosis, but is not refunded. Until recently, hemp was also available in Poland in a dried form containing 19% THC and <1% CBD. This medicine does not have the characteristics of a medicinal product, and thus the specific indications for its use cannot be listed. The dried form is registered as a prescription ingredient. Some studies have shown that cannabinoids might reduce the severity of nausea and vomiting associated with chemotherapy ^[20][34] and carry analgesic effects ^[21][35]. THC and CBD have alleviated acute and chronic pain in animal studies, but results in cancer patients are inconclusive ^[22][36]. A small study reported improved anxiety, mood, and well-being with cannabinoids in cancer patients ^[23][11]. However, no extensive clinical trials have been undertaken to confirm the analgesic effect of THC and CBD ^[24][69]. Anticancer effects of cannabinoids have only been tested on cancer cell lines. The administration of CBD increased the sensitivity of multiple glioma cells to chemotherapy ^[25][10]. Antiproliferative effects of THC and CBD have also been shown in breast, uterus, gastric, colorectal, pancreatic, lung adenocarcinoma, prostate cancer, and lymphoma cell lines ^[23][11]. The side effects of cannabinoids are primarily related to their stimulant and depressant effects on the CNS. Patients may experience confusion, impaired memory, drowsiness, and perceptual disturbances. Interestingly, unlike opioid receptors, cannabinoid receptors are not present in the brain’s respiratory center. Thus, in the case of cannabinoid overdose, there is no fear of respiratory depression ^[23][11]. However, THC and CBD affect receptors not only in the nervous system; they can cause tachycardia, hypotension, muscle relaxation, or impaired gastrointestinal motility ^[26][51]. The risk of cannabinoid addiction is lower than for tobacco, alcohol, or cocaine. Withdrawal symptoms such as irritability, restlessness, nausea, or insomnia have been less severe than those accompanying benzodiazepines or opiates, and usually resolve after a few days ^[23][11].

5. Propolis

Propolis is a resinous substance collected by bees from the buds and shoots of young trees and green plants. It is available as a dietary supplement in several forms, including pills, capsules, tablets, drops, syrups, ointments, sprays, powders, or liquids for skin application. Studies on cell lines and animal models have shown a cytotoxic effect of propolis on breast, cervical, skin, gastric, prostate, and leukemia cancer cells, and a protective effect on the DNA of healthy cells ^{[27][28][29][30]}[12,13,14,15]. A Polish study claimed that ethanol extract of propolis has cytotoxic activity on glioblastoma cell lines ^[31][70]. Due to its anti-inflammatory and antimicrobial effects, propolis extract was also used to treat complications after radiotherapy. A small study showed that propolis extract allowed the healing of radiation skin ulcers resistant to standard treatments ^[32][71]. Finally, in a study involving over 200 patients with breast cancer and head and neck cancer, propolis solution was shown to be effective and safe in preventing and treating oral mucositis caused by radiotherapy or chemotherapy ^[33][37]. However, larger clinical studies have not confirmed any beneficial effects in patients with radiation or postoperative ulcers and oral mucositis. So far, no study has shown the anticancer effects of propolis.

6. Vitamin C

Vitamin C is one of the most potent antioxidants. The first reports of its potential anticancer effects were published in the 1970s. An increasing number of institutions offer intravenous vitamin C infusions for cancer patients, advertising them as an adjunctive or anticancer modality. High doses of vitamin C inhibit the growth of prostate, colon, and pancreatic cancer, as well as mesothelioma cell lines ^{[34][35][36][37]}[16,17,18,19]. In a phase I study, the addition of high vitamin C doses to anticancer therapies (e.g., gemcitabine) was claimed to increase their effectiveness ^[37][19]. However, in other studies conducted on cell lines and in animal models, high doses of vitamin C reduced the effectiveness of chemotherapy ^[38][39][40][38,52,72]. In a small study, intravenous vitamin C infusions reduced chemotherapy-related symptoms such as fatigue, nausea, vomiting, or loss of appetite [38]. Currently, there is no evidence to confirm the beneficial effects of intravenous vitamin C in cancer patients, and its use may even reduce the effectiveness of treatment. Due to the risk of hemolysis, intravenous vitamin C is contraindicated in patients with glucose-6-phosphate dehydrogenase deficiency ^[41][53], and even its oral form in these patients should be used with caution. High doses of vitamin C should not be administered in patients with a predisposition to kidney stones ^[38][39][38,52]. Vitamin C infusions administered shortly before chemotherapy may cause adverse interactions ^[39][52].

7. Turmeric

Turmeric is a spice originating from India and has been used in traditional Chinese and Ayurvedic medicine since ancient times. This compound has attracted great interest in recent decades because it contains bioactive curcuminoids (curcumin, demethoxycurcumin, and bisdemethoxycurcumin). Laboratory studies have shown its antioxidant and anti-inflammatory effects ^[42][43][20,21]. Turmeric was shown to increase the sensitivity of cancer cells to cisplatin, 5-fluorouracil, paclitaxel, and radiotherapy ^{[43][44][45][46][47]}[21,22,23,24,73]. Some studies also demonstrated a chemoprotective effect of turmeric against the development of head and neck cancer and colorectal cancer ^[48][49][74,75]. The clinical data on turmeric are scarce. In patients with colorectal cancer, turmeric reduced weight loss and decreased serum inflammatory parameters ^[50][39]. A phase II trial involving 44 patients claimed that 30-day turmeric therapy might reduce tumor size ^[51][40]. Other studies involving small groups of patients with prostate and pancreatic cancer, that investigated this compound alone or combined with radiotherapy and chemotherapy, have been inconclusive. In patients with head and neck cancer and breast cancer, turmeric reduced radiation skin reactions ^[52][53][76,77]. No severe side effects of turmeric have been observed, but there are interactions with antiplatelet agents [54], doxorubicin, or tacrolimus [55]. Turmeric also affects the activity of cytochrome P450 enzymes [56].