Is drinking coffee or tea against the fifth precept?

I think that the Heedlessness idea is on track. The Buddha didn't warn against cocaine, heroin or methamphetamine, but they can clearly be included in the 5th Precept these days. I think the issue would be how do coffee and tea affect you? You might abstain to see. You might have them for a reason. I don't hold absolutely with the "addiction" line because we are all addicted to sunlight, air, water and food. More or less of any of them might help or harm. Coffee and tea are no more inherently harmful than any of these. The fact that 90% of the worlds people use caffeine on a daily basis should tell us something. (What should it tell us?)

Black tea is fermented, by the way. Green tea is not.

Tea or coffee drinking is not an issue. This is well explained in "THE FIVE PRECEPTS" which is a very good read on the 5 precepts.

The fifth precept reads: Surā-meraya-majja-pamāda-ṭṭhānā veramaṇī sikkhāpadaṃ samādiyāmi, “I undertake the training rule to abstain from fermented and distilled intoxicants which are the basis for heedlessness.” The word meraya means fermented liquors, sura liquors which have been distilled to increase their strength and flavor. The world majja, meaning an intoxicant, can be related to the rest of the passage either as qualified by sura,meraya or as additional to them. In the former case the whole phrase means fermented and distilled liquors which are intoxicants, in the latter it means fermented and distilled liquors and other intoxicants. If this second reading is adopted the precept would explicitly include intoxicating drugs used non-medicinally, such as the opiates, hemp, and psychedelics. But even on the first reading the precept implicitly proscribes these drugs by way of its guiding purpose, which is to prevent heedlessness caused by the taking of intoxicating substances.

A more literal interpretation is, for example, from Bhikkhu Bodhi

"The fifth precept reads: Suramerayamajjapamadatthana veramani sikkhapadam samadiyami, "I undertake the training rule to abstain from fermented and distilled intoxicants which are the basis for heedlessness."

Coffee & tea, not being fermented or distilled intoxicants wouldn't break the 5th precept I wouldn't think. Of course limiting caffeine and sugar and lots of other stuff will help keep your mind calm and makes sense for good health and meditation.

The five precepts are an entry-level practice that is designed to help practitioner master basic discipline of self-reflection and self-control.

Abstaining from killing, stealing etc. implies basic ability to watch one's mind for harmful thoughts and emotions, and to prevent them from getting acted out.

In light of the above, the no-killing rule should not be understood literally, as an absolute law, but as a guideline for training the mind. It is not acceptable to hurt or harm any creature (big or small) out of antipathy. This is what the no-killing rule is mostly about, countering the first poison, usually translated Anger or Hate but really meaning Aversion/Antipathy/Negativity.

Similarly, the no-stealing rule is mainly about countering another of Three Poisons, that of Obsessive Attraction or Desire.

Likewise, the no-intoxication rule is meant to counter the habitual mind-state of Ignorance (obscurity, confusion). Many people prefer to get drunk or stoned as a way to anesthetize themselves. They don't seem to see the connection between the dimmed mind and the causes of suffering. The no-intoxication rule is meant to guide people out of their comfortable haze and towards lucidity of enlightenment. In this vein, watching TV or playing videogames, is too a form of intoxication and should be minimized.

Drinking tea or coffee does not produce haze and therefore is not subject to the fifth precept. That said, being addicted to e.g. coffee is not much different from any other addiction in that produces a state of obsessive desire.

The literal meaing there is avoiding fermented drink that causes heedlessness.

Surā-meraya-majja-pamāda-ṭṭhānā veramaṇī sikkhāpadaṃ samādiyāmi

I undertake the precept to avoid [that big long word]

Big long word happily rendered as "beer-cider-carelessness-intoxication-condition" by Kare.

This is talking specifically about fermented things like beer, wine, etc. Neither coffee nor tea are fermented, nor do they cause heedlessness. I've heard of monks drinking tea. (I've heard of monks smoking too, which I think isn't forbidden by this rule, although perhaps by others)

It's all well and good to look past the behavior prohibited by the precepts to see what further kinds of attachments you have that can be abandoned, but not, I think, at the cost of going back to change the meaning of the precept.

But I do occasionally see attempts at redefining the precepts to be stricter than they are. Take Sulak Sivaraksa's reinterpretation of the five precepts:

Sulak reinterprets the five classic Buddhist precepts for the modern day.

1. Individuals may not be killing outright, but they must examine how their actions might support wars, racial conflict, or the breeding of animals for human consumption.

2. Considering the second precept of abstaining from stealing, Sulak questions the moral implications of capitalism.

3. Stopping exploitation of women is a natural extension of the third precept of abstaining from sexual misconduct.

4. And vowing to abstain from false speech would naturally bring into question how mass media and education promote a biased view of the world.

5. Finally, the fifth precept to avoid intoxicants deals with international peace and justice because, “the Third World farmers grow heroin, coca, coffee, and tobacco because the economic system makes it impossible for them to support themselves growing rice and vegetables.

All good points and things worth thinking about, but these are going beyond the five precepts. The precepts are intended to take you to a point where you can examine all of your behavior to see what is skillful, what leads to affliction, etc.

But the precepts shouldn't be redefined to also include these new things. This would only lead to divisiveness between people as one person preaches getting rid of coffee and others disagree. Remember the Buddha refused to make vegetarianism mandatory for the sangha, even though the killing involved in eating meat can easily be seen to pertain to the first precept.

In the case of coffee, I could see a person getting riled up by it and possibly being less able to control their mind, being quicker to anger, etc. If this is the case, then it should certainly be examined, and perhaps that behavior should be abandoned, but this is not covered by the fifth precept.