Such a question should perhaps be rephrased as, 'how much caffeine is harmful?'
In most people between 200-300mg of caffeine is not harmful. This equates to about 3-4 cups of coffee a day.
However, given a wide range in peoples sensitivity to caffeine, unwanted negative effects of caffeine may occur in amounts much less or greater than this.
Given differences in the variety and types of coffee available it is not possible to give an accurate figure for the amount of caffeine in a typical cup of coffee.
However, as a guide, one serving (6oz/ 178ml) of normal strength instant coffee contains about 30mg of caffeine (with a range anywhere between 10-35mg).
Brewed coffee tends to have a slightly higher level of caffeine with a 6 oz serving containing 40mg of caffeine (with a range between 30-90mg).
No. There is a misconception that apples contain caffeine when in fact they do not.
Apples do not contain caffeine, see above.
When withdrawing from caffeine, there are two approaches.
The first approach is stop taking caffeine immediately; this method is also known as 'cold turkey'. A person may experience unpleasant effects of withdrawal including possible headaches, irritability, depression and disrupted sleep for a few days. Although not strictly 'cold turkey', these effects may be lessened to taking a low dose of caffeine once withdrawal symptoms arise to take the edge off them.
The second method of withdrawing from caffeine is to gradually reduce caffeine intake. For example, a person consuming 5 cups of coffee a day might reduce this down by 1 cup every couple of days and after a week or so stop completely.
As decaffeinated coffee and other drinks (e.g. decaffeinated tea) usually contain low levels of caffeine, substituting decaffeinated coffee for normal strength coffee can be an effective way to cut down on caffeine intake whilst keeping withdrawal symptoms at bay.
However, be aware that due to low levels of caffeine still present in many decaffeinated products, it may be possible to maintain an addiction to decaffeinated products; withdrawal symptoms may be felt after stopping them.
Once caffeine intake has been stopped, it may be helpful to replace the previous source of caffeine (e.g. a serving of coffee) with an alternative such as a herbal tea which does not contain caffeine.
This may help satisfy any habitual craving associated with regular daily consumption of caffeine whilst avoiding caffeine intake; such cravings are temporary and should pass with time.