Scientists believe compounds called histone deacetylase inhibitors (HDACis) allow the brain to "override" disturbing recollections with new non-threatening memories.
They hope the drugs, so far only tested on mice, can be used to clear away frightening memories anchored too firmly to be dislodged by conventional therapy.
The aim is to develop new treatments for post-traumatic stress disorder (PTSD), which affects a third of people who have had a traumatic experience such as military combat, a violent attack, or being involved in a major accident or disaster.
PTSD is marked by nightmares, flashbacks, feelings of isolation, poor sleep and "jumpiness".
A common form of psychotherapy treatment involves helping sufferers "re-live" their traumatic memory under controlled conditions. This process, known as exposure-based therapy, opens a short window that allows the memory to be disrupted and replaced by new memories.
However, it tends not to work for older, more entrenched memories.
The new research suggests that HDAC is make the brain more malleable, enabling even deep-rooted traumatic memories to extinguished by exposure-based therapy.
Lead scientist Dr Li-Huei Tsai, from the Massachusetts Institute of Technology said: "This study provides a mechanism explaining why old memories are difficult to extinguish and shows that HDACis can facilitate psychotherapy to treat anxiety disorders such as PTSD."