None. There are no medications that are used specifically to promote twins. Multiples are far more risky. About 2% of pregnancies (not involving infertility treatments) result in multiple gestations - almost all of them are twins - triplets occur about once in 10, 000 pregnancies. The chance of twins is much greater when on fertility drugs or with other assisted reproductive treatments such as ivf.
Want twins? You indicate that you want twins, and many of my pts want them too. It is not a good idea though because towns are complicated pregnancies because of possible premature birth. Prematurity is a problem for the babies more than the mom. If you try to get twins then you could end up with triplets or worse. Multiples are a problem we have with fertility treatments and we want less. Have 1 at a time :).
Fertility drugs. Multiple pregnancy is a risk with all fertility drugs including Clomid (clomiphene) and Letrozole (femara). Although most twins do fine, the risk of perinatal death and cerebral palsy (as well as the cost per pregnancy) is much higher with twins compared with singletons. The risks and costs are greatly increased with triplets and more. The goal is one child at a time.