There has not been enough research to sufficiently determine whether or not the medicinal use of blackberry juice might have side effects. However, blackberry juice used at levels consistent with typical food and drink portions is currently considered completely safe.

At the time of this writing, the only known blackberry juice side effects that should cause concern are the slight possibilities of nausea and vomiting. This is due to the relatively low level of tannins present in the fruit. Most people will not experience such side effects, but those with sensitive stomachs and a history of chronic gastrointestinal problems should use blackberry juice with caution.

As long as nausea or vomiting are not experienced, there is no evidence to suggest that a person can drink too much blackberry juice.