An effort to essentially decriminalize the use of marijuana for medical purposes won preliminary approval yesterday in the Maryland House of Delegates as members rejected a string of amendments that proponents said would have effectively killed the bill. The legislation, which would significantly reduce the penalties for possessing or using marijuana, could come for a final vote as early as next week. A companion bill in the Senate concerning medical marijuana use is in committee. Gov. Robert L. Ehrlich Jr. (R) has signaled that he could support such a measure.