Northern Ireland authorities are waiting for results of forensic tests to help determine if the recent deaths of eight people in their 20s and 30s were caused by rogue ecstasy tablets.
A warning has been issued about a batch of green pills, with a crown or castle logo.
The cause of death for seven people in the greater Belfast area and one from the North West has not yet been established.
However, a letter to health professionals from Northern Ireland's chief medical officer Dr Michael McBride raised concerns.
It noted the possibility that individuals may have taken what they thought was a weak ecstasy table but was, in fact, a highly toxic substance.
More information will be made available when the tests are completed.