▁as ▁p eth id ine ▁( me per id ine ) ▁and ▁de xt ro pro po xy ph ene . ▁ ▁Pro dru gs ▁▁ ▁Sometimes ▁dru gs ▁are ▁form ulated ▁in ▁an ▁in active ▁form ▁that ▁is ▁designed ▁to ▁break ▁down ▁inside ▁the ▁body ▁to ▁form ▁the ▁active ▁drug . ▁These ▁are ▁called ▁prod ru gs . ▁The ▁reasons ▁for ▁this ▁type ▁of ▁form ulation ▁may ▁be ▁because ▁the ▁drug ▁is ▁more ▁stable ▁during ▁manufact ure ▁and ▁storage ▁as ▁the ▁prod rug ▁form , ▁or ▁because ▁the ▁prod rug ▁is ▁better ▁absor bed ▁by ▁the ▁body ▁or ▁has ▁superior ▁ph arm ac ok inet ics ▁( e . g ., ▁l is dex am ph et am ine ). ▁ ▁References ▁ ▁Further ▁reading ▁▁▁ ▁Category : Ph arm ac ok inet ics ▁Category : Met abol ism <0x0A> </s> ▁The ▁Ir eng ▁River ▁( or ▁Ma ú ▁River ) ▁forms ▁part ▁of ▁Guy ana ' s ▁western ▁border ▁with ▁Brazil . ▁ ▁It ▁flows ▁through ▁the ▁valle ys ▁of ▁the ▁Pak ara ima ▁Mountains ▁for ▁most ▁of ▁its ▁length . ▁It ▁is ▁the ▁only ▁major ▁river ▁in ▁Guy ana ▁which ▁flows ▁from ▁North ▁to ▁South . ▁It ▁is ▁one ▁of ▁the ▁northern most ▁trib ut aries ▁of ▁the ▁Amazon ▁river ▁system . ▁ ▁The ▁sources ▁of ▁the ▁river ▁are ▁in ▁the ▁ ▁Monte ▁R ora ima ▁National ▁Park , ▁created ▁in ▁ 1 9 8 9 . ▁The ▁larger ▁part ▁of ▁the ▁Ir eng ▁River ▁bas in ▁forms ▁the ▁front