There are many potential causes of short-term memory loss, and many of them are reversible. Memory loss may be a side effect of medication (or a combination of medications). It can occur after a head injury or as a result of vitamin B-12 deficiency. Hypothyroidism (an underactive thyroid gland) can affect memory. So can stress, anxiety, depression, and alcohol use. Or, memory loss could be a symptom of a serious condition, such as dementia or a brain tumor.