The present invention describes new homo or heterodimer prodrugs wherein both active monomeric units are compounds of the formula: ##STR00001## where R.sub.1 and R.sub.2 are each independently --H, --C.sub.1 C.sub.6 alkyl --C.sub.1 C.sub.6OH, or --C.sub.1 C.sub.6NHR', wherein R' is --H, or --C.sub.1 C.sub.6 alkyl, R.sub.3 is: ##STR00002## where R.sub.4 and R.sub.5 are defined in the description. The monomeric units are coupled to each other via a labile carbonate (--O--COO--), carbamate (--O--CO--N--), urea (--N--CO--N--) or phosphate (--O--POO--O--) moiety. Such homo or heterodimer prodrugs are useful in the treatment of erectile dysfunction.