The tablets are red or pink, reports Swedish television SVT.
There have also been three overdoses linked to these pills.
But Stockholm police are not sure whether the "Superman" tablets are the cause, or whether they are incidental and there is some other factor.
Police spokesperson Annika Ljung says parents should be on their guard and check whether their children have ordered anything unusual off the internet.
Ecstasy pills are illegal in Sweden, where policy is to eradicate drug use through criminal convictions and medical treatment.