A cup of coffee each morning may wake you up, but a new study suggests caffeine might hinder your short-term recall of certain words.
Caffeine found in two strong cups impaired word recall
Caffeine made it harder for people to find a word that they already knew - the "tip-of-the-tongue" phenomenon.
Valerie Lesk, of the International School for Advanced Studies in Italy, believes caffeine improves alertness by shutting down other brain pathways.
This makes it harder to recall words, she says in Behavioural Psychology.
Caffeine is known to excite the brain and increase alertness.
But Miss Lesk and her colleague Stephen Womble, from Trinity College, Dublin, found it can hamper or boost short-term memory, depending on what you are trying to remember.
They divided 32 college students into two groups. One group was given 200mg of caffeine, which is equivalent to two strong cups of coffee, and the other was given a dummy drug.
The students were then asked to answer 100 general knowledge questions that had simple, one-word answers.
For example, one question was "Name the ancient Egyptian writing," with a target answer of "hieroglyphics".
For each question the student was given 10 words to look at before answering.
Between two and eight of the words were similar sounding to the answer, for example hierarchy sounds similar to hieroglyphics.
The other words were completely unrelated to the answer.
If the student was unable to answer the question, the researchers gave them a prompt by sounding out the beginning of the answer - for example hi for hieroglyphics.
If the student was then able to give the correct answer after prompting this was recorded as a "tip-of-the-tongue" episode.
Caffeine aided word recall when the words the students looked at were similar to the answer.
But when the words were unrelated to the answer, the students who had taken caffeine had more trouble recalling the answer than those who had taken the dummy pill.
Miss Lesk said: "In some conditions caffeine helps short-term memory and in others it makes it worse.
"It aids short-term memory when the information to be recalled is related to the current train of thought but hinders short-term memory when it is unrelated
"If the word is unrelated then caffeine is still strengthening retrieval in the same way, but because it is unrelated to the word you want to find it is actually having a negative effect," she said.
The researchers are investigating the effect of caffeine in people with word retrieval difficulties, a condition called aphasia.
A spokeswoman from the British Coffee Association said the study was too small to be able to generalise.
"There are many studies that show quite clearly the beneficial effects that moderate caffeine consumption can have on alertness, cognitive function and mood. This one study does not rewrite the science," she said.