The present invention generally relates to the transdermal delivery of various compounds. In some aspects, transdermal delivery may be facilitated by the use of a hostile biophysical environment. One set of embodiments provides a composition for topical delivery comprising a phosphodiesterase type 5 inhibitor and/or a salt thereof, and optionally, a hostile biophysical environment and/or a nitric oxide donor. In some cases, the composition may be stabilized using a combination of a stabilization polymer (such as xanthan gum, KELTROL® BT and/or KELTROL® RD), propylene glycol, and a polysorbate surfactant such as Polysorbate 20, which combination unexpectedly provides temperature stability to the composition, e.g., at elevated temperatures such as at least 40° C. (at least about 104° F.), as compared to compositions lacking one or more of these.