There are some very specialised therapeutic peptides that can be taken orally but these have a special formulation which protects them from digestion
Liu S, Shi J, Wang L, Huang Y, Zhao B, Ding H, et al
On the other hand, glutathione conjugates of certain substances may be further metabolized into more toxic products
Systemic allergic reactions are rare
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A big study in the US, Canada, and Australia found links to depression, anxiety, and suicidal thoughts