The side effects of astragalus are the worsening of autoimmune diseases such as rheumatoid arthritis and lupus, according to WebMD. This is because astragalus strengthens the immune system, and in autoimmune diseases the immune system attacks the body.
Astragalus should not be taken with corticosteroids or drugs taken to suppress the immune system, says the University of Maryland Medical Center. It should not be taken with drugs taken by transplant patients to lower the risk of rejection, nor should it be taken with lithium. Astragalus makes it harder for the body to excrete lithium. Medical professionals do not know if the herb is safe for nursing mothers, as of 2015.
Blood pressure and blood glucose levels may also be affected by astragalus, claims the National Institutes of Health. People who take astragalus must also be careful of the species of the herb because some species contain the toxin swainsonine. This causes a condition called locoism in livestock, according to Cornell University.
Other types of astragalus have such high levels of selenium that they too are dangerous to ingest, claims the National Institutes of Health. Though selenium is necessary for life in trace amounts, selenium poisoning leads to diarrhea, nausea, fatigue, hair loss and joint pain. It also leads to discoloration, brittleness and loss of fingernails.