In an article published on the website of the world's foremost scientific journal, Nature, possible side effects of tongkat ali (Eurycoma longifolia) were listed as "a light headache, a slight fever or even a rash" for a few days. Referred-to results were "increased sexual desire, enhanced performance and general well-being".
As usually is the case with scientific articles, a payment is required to read the full article. However, an abstract can be read for free on the Nature website here:
http://www.nature.com/nm/journal/v11/n9/full/ nm0905-912a.html
References:
George A, Udani J, Abidin NZ, Yusof A. (2018).
Efficacy and safety of Eurycoma longifolia (Physta®) water extract plus multivitamins on quality of life, mood and stress: a randomized placebo-controlled and parallel study. Food Nutrition Resources, Vol 62.
Henkel R.R.; Wang R.; Bassett S.H.; Chen T.; Liu N.; Zhu Y. et al. (2014),
Tongkat Ali as a potential herbal supplement for physically active male and female seniors - A pilot study , Pharmacological Research,
Volume 28, Pages 544-550, https://doi.org/10.1002/ptr.5017
Hien, Dao Thi Thanh; Long, Tran Phi; Thao, Tran Phuong; Lee, Jeong-Hyung; Trang, Duong Thu; Minh, Nguyen Thi Thu; Cuong, Pham Van; Lan, Dang, Nguyen Hai; Dat, Nguyen Tien (2019), Anti-inflammatory effects of alkaloid enriched extract from roots of Eurycoma longifolia Jack, Asian Pacific Journal of Tropical Biomedicine, Volume : 9 , Issue : 1 , Pages : 18-23
Nhan NH, Loc NH. (2017), Production of eurycomanone from cell suspension culture of Eurycoma longifolia , Pharmaceutical Biology Vol 55(1) Pages 2234-2239
Rehman SU, Choe K, Yoo HH. (2016), Review on a Traditional Herbal Medicine, Eurycoma longifolia Jack (Tongkat Ali): Its Traditional Uses, Chemistry, Evidence-Based Pharmacology and Toxicology , Molecules, Vol 21(3) Pages 331
Ruan, Jingya; Li, Zheng; Zhang, Ying; Chen, Yue; Liu, Mengyang; Han, Lifeng; Zhang, Yi; Wang, Tao (2019), Bioactive Constituents from the Roots of Eurycoma longifolia, Molecules, Volume 24(17), Pages 3157; https://doi.org/10.3390/molecules24173157
Tambi, Ismail (2021), Herbal medicine used to treat andrological problems: Asia and Indian subcontinent: Tongkat Ali (Eurycoma longifolia jack) , Herbal Medicine in Andrology
An Evidence-Based Update, Pages 107-111
Udani J.K.; George A.A.; Musthapa M.; Pakdaman M.N.; Abas A. (2014), Effects of a proprietary freeze-dried water extract of Eurycoma longifolia (Physta) and Polygonum minus on sexual performance and well-being in men: a randomized, double-blind, placebo-controlled study, Evidence-Based Complementary and Alternative Medicine, https://doi.org/10.1155/2014/179529
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