Over at alt.healing.reiki Fascin8or posted an interesting reply about energy leakages and natural vampires:
[...] training or not, we are all constantly (though usually unconsciously) 'tapping into', interacting with and expending our own personal ki as part of the very process of living. [There is a very specific term used to describe anyone does not do so, and that term is: dead] Fortunately, most of the time our system is also capable of sufficiently replenishing our personal ki levels as a part of same natural process of living.
Every time we consciously focus on our breath, we are tapping into our personal ki - and while certain breathing rhythms (whether unconsciously occurring , or consciously activated) help us absorb external ki - thereby replenishing our levels of personal ki, other rhythms actually cause depletion and dispersal of our personal ki.
Some folk (i.e. those poetically referred to as either 'psychic vampires' or 'energy vampires') seem possessed of a natural ability to 'leech' personal energy from others; and on the flip side of the coin, there are many from whom personal ki simply 'escapes' under certain circumstances. Fortunately, in most cases this leakage is akin to an energetic 'slow puncture' and while debilitating, is not too serious - providing the episodes of leakage are not too frequent or long-lasting - thus giving the individual time to recover.
Throughout history, such 'bleeders' (especially those who have come to a realisation of what is happening and find their own ways to at least partially control it) have frequently become successful healers [successful healers-of-others that is, though commonly, over the long-term, to the detriment of their own health].
Even non-'bleeders' can (without any training) give of their own personal ki, if the desire or intent to do so is strong.
And how many of us have come across situations in which novice Reiki students find themselves tired or drained after giving treatment, simply because in their eagerness to achieve, they are trying too hard (not yet being comfortable with the concept of simply letting go and letting the Reiki happen) - they feel the need to 'do', to push the energy - and in doing, so are adding their own personal energy into the flow - hence the feeling of tiredness.