The invention provides methods and compositions for treating erectile dysfunction. The methods include the step of placing within the fossa navicularis of the patient an effective erection-inducing amount of a vasoactive prostaglandin composition of a semi-solid consistency. The composition comprises a vasoactive prostaglandin, a penetration enhancer, a shear-thinning polysaccharide, a lipophilic compound, and an acidic buffer system. The penetration enhancer is an alkyl-2-(N-substituted amino)-alkanoate ester, a (N-substituted amino)-alkanol alkanoate, or a mixture of these enhancers. The lipophilic compound may be an aliphatic C.sub.1 to C.sub.8 alcohol, aliphatic C.sub.2 to C.sub.30 ester, an aliphatic C.sub.8 to C.sub.30 ester, or a mixture of these. The composition includes a buffer system providing a buffered pH value for said composition in the range of about 3 to about 7.4.