Medical experts caution against pregnant women using ashwagandha because it may cause loss of the fetus. This is supported by the traditional use of ashwagandha as an abortificant. Another stated precaution is that ashwagandha may potentially increase the sedative effect of barbiturate drugs.4
The only report of a potentially severe adverse side effect possibly attributable to ashwagandha use was a case of sudden increase in thyroid hormone levels in a woman taking ashwagandha supplements for fatigue.4
Although it was once believed that ashwagandha contained nicotine, this has since been refuted.5