When it comes to pro-sexual effects, in terms of popularity and fame only, there are the PDE5 inhibitors. The main examples are sildenafil (Viagra) and Tadalafei Cialis.
These drugs inhibit the breakdown of cGMP in blood vessels filling the penis, providing some increase in blood flow relative to when the drug is not used.
So, the user gets a somewhat more pro-sexual effect than what he would have had anyway without the drug. One way of putting it is that the quality of the erection may increase by about a letter grade. A “B” grade erection would likely improve to an “A,” or a “C” grade erection might improve to a “B” or even a “B+.”
PDE5 inhibitors provide no increase in libido. The body can adapt to them, so I would avoid frequent use.
The melanocortin agonists are much more interesting. The principal examples are Melanotan II and bremelanotide (PT-141.) These are injectable peptides that are available as research chemicals. Opposite to the PDE5 inibitors, they do nothing directly at the penis, but act at the brain to activate the nervous system. Erections typically are much more readily produced, and desire can increase.
Pro-sexual effect generally begins at least several hours after injection. The delay may be as much as 8 hours after injection, as an estimate.
Melanotan II also promotes tanning, and in some can increase or darken moles. Effective dose is about 1-2 mg. Many users will tan much darker from even only 30 mg total of Melanotan II or less.
PT-141 has no or little effect on tanning, but may give side effects of nausea or backache. Effective dose ranges from about 0.5 mg to 2 mg.
For some, drugs which increase dopamine are effective prosexuals. Of these, in my opinion the safest and most healthful choice is selegiline (Deprenyl), typically at 2.5 mg/day.
Pramipexole (Mirapex) is another choice. This drug requires more care. A typical safe dose is 0.25 mg prior to bedtime; in my opinion more than 0.5 mg should never be used except when treating disease.
Unlike the PDE5 inhibitors and melanocortin agonists, dopaminergic drugs have better prosexual effect when used every day rather than on specific occasion.
For stacking, I would use no more than one drug from each class. Compounds may be selected from any two or all three of these classes.
Also, where estradiol levels are too high, an aromatase inhibitor can have prosexual effect by correcting this. Where testosterone or estradiol is low, often HCG can provide correction, which again may improve sexual function.
(HCG increases estradiol only a small amount, via increasing testosterone, but the amount can be enough to increase estradiol levels from below-normal to low-normal.)
It could be more important to correct underlying problems in these areas than to seek the above drugs.