Two major drug types affect memory majorly.

1) Benzodiazepines, like Valium, Xanax etc will cause memory loss. They are used in anesthesias for dental work, because of their amnesiac effects. That link to the rat study Dr. Smith put up explains the GABA system and how that works. Benzos are commonly given with opiates to enhance effects, so many people use them and also for muscle relaxing effects.

2) Statins for lowering cholesterol, now have a warning on them for affecting memory. So if you use them you might discuss this with your doctor. Statins damage neurons. There are now photos of this effect published this past year.

Any drug that causes mental confusion, sedation, can affect memory too. In the elderly common acid blocking drugs, Zantac and Pepcid, can enter the brain more easily thru the blood brain barrier and cause confusion and in some delirium.