Some supplements you maybe should not take


As there is a possibility of harmful effects on your health from certain supplements, I would recommend against taking any of the following nutrients as a supplement (single-nutrient supplement or a supplement containing these nutrients) ... unless, of course, a medical doctor has prescribed them for you (see below).
  • Vitamin A --- may increase cancer risk (1, 2)
  • Beta-carotene --- might increase all-cause mortality (and lung cancer risk in smokers) (1
  • Vitamin E --- may increase the risk of haemorrhagic stroke (2)
  • Choline --- might increase blood TMAO levels which might increase cardiovascular risk (3)
  • High-dose calcium --- might increase the risk of stroke, heart attack, and kidney stones (4, 5); low-dose calcium supplements, i.e. around 300 mg per day, might be harmless (6); combined supplements of vitamin D plus calcium might even decrease lung cancer risk (7).
  • Iron --- might lead to high iron stores which may then damage the heart (and other organs) through generation of free radicals and so-called ferroptosis (8, 9); this is controversial, however, and the mechanisms of iron metabolism are complicated (10, 11, 12, 13). So, it is possible but not certain that iron supplements, when taken by non-iron-deficient individuals, could increase the risk of cardiovascular disease and other chronic diseases.
If a medical doctor has prescribed a certain supplement for you, the situation is different: for example, if you have iron deficiency, a medical doctor may advise you to take an iron supplement (14-19).

Of course, this is not a complete list of supplements which may potentially be harmful. But the nutrients above are sometimes recommended by some vegans - and this may not be a good idea.




References:

1) Schwingshackl et al. 2017 https://pubmed.ncbi.nlm.nih.gov/28096125/ doi: 10.3945/an.116.013516
2) Cortés Jofré et al. 2020 https://www.ncbi.nlm.nih.gov/pubmed/32130738 doi: 10.1002/14651858.CD002141.pub3
3) Cho et al. 2020 https://pubmed.ncbi.nlm.nih.gov/32722424/ doi: 10.3390/nu12082220
4) Khan et al. 2019 https://pubmed.ncbi.nlm.nih.gov/31284304/ doi: 10.7326/M19-0341
5) Chiodini and Bolland 2018 https://pubmed.ncbi.nlm.nih.gov/29440373/ doi: 10.1530/EJE-18-0113
6) Zhang et al. 2021 https://pubmed.ncbi.nlm.nih.gov/33491043/ doi: 10.1007/s12603-020-1551-9
7) Sun et al. 2020 https://pubmed.ncbi.nlm.nih.gov/33225749/ doi: 10.1080/01635581.2020.1850812
8) Kobayashi et al. 2018 https://pubmed.ncbi.nlm.nih.gov/29874997/ doi: 10.2174/1389450119666180605112235
9) Naito et al. 2021 https://pubmed.ncbi.nlm.nih.gov/32739111/ doi: 10.1016/j.jjcc.2020.07.009
10) DeLoughery 2019 https://pubmed.ncbi.nlm.nih.gov/30970354/ doi: 10.1159/000496966
11) Cornelissen 2019 https://pubmed.ncbi.nlm.nih.gov/31416722/ doi: 10.1016/j.ebiom.2019.08.014
12) Ravingerová et al. 2020 https://pubmed.ncbi.nlm.nih.gov/33114290/ doi: 10.3390/ijms21217889
13) Ying et al. https://pubmed.ncbi.nlm.nih.gov/33978777/ doi: 10.1007/s00059-021-05039-w
14) Zhang et al. 2019 https://pubmed.ncbi.nlm.nih.gov/30782220/ doi: 10.1017/S000711451900014X
15) Stoffel et al. 2020 https://pubmed.ncbi.nlm.nih.gov/32650997/ doi: 10.1016/j.mam.2020.100865
16) Pedlar et al. 2018 https://pubmed.ncbi.nlm.nih.gov/29280410/ doi: 10.1080/17461391.2017.1416178
17) Houston et al. 2018 https://pubmed.ncbi.nlm.nih.gov/29626044/ doi: 10.1136/bmjopen-2017-019240 
18) Agoro and Mura 2019 https://pubmed.ncbi.nlm.nih.gov/31108902/ doi: 10.3390/ph12020075
19) Del Vecchio et al. 2020 https://pubmed.ncbi.nlm.nih.gov/34188903/ doi: 10.1093/ckj/sfaa212