WHEN I came to from the operation I was vaguely aware of voices and then of someone speaking directly to me: 'Don't forget to press the button for painkillers, David.'

A hand touched mine and drew my attention to the control clutched within my fingers.

I have recently undergone an operation to remove my large intestine because of bowel cancer, and was fortunate enough to have the use of patient-controlled analgesia (PCA) after surgery. In the hours and days after my operation, I was able to control my own pain relief, a system that is of enormous benefit to the patient - and apparently saves the NHS money in the long run.

The practicalities were explained on the morning of the operation. A small unit is clamped to a stand by the bed, comprised of a syringe of analgesic (a pain-relieving drug), in my case morphine, with a fine tube into a vein in the back of the hand. A cable with a button on the end is placed in the other hand. When you press the button the unit delivers a shot directly into the bloodstream.

The first time I did so, something bleeped at me. The machine was working. Gingerly I allowed my mind to explore my body. I was not hurting. To be sure, I was aching. The whole of my insides ached. But it was tolerable. My fear had been that I would wake up nailed to a bed of excruciating pain. This ache was not at all what I'd been dreading.

I drifted through the night. Nurses checked my temperature, pulse, blood pressure. Every now and then I pressed the button, and the machine bleeped reassuringly back at me. The ache in my belly was always there, but it didn't dominate my existence.

The night, however, lasted an eternity. This slow passage of time was one effect of the morphine that I was not prepared for. I watched the moon crawl across the sky, wondering if it would ever disappear behind the hill at the other end of the window. Mornings and afternoons seemed the length of a day.

Some time the next morning I was told I was going to have a bed-bath and would be helped into an armchair while my bed was made. This is going to hurt, I thought, and pressed the button for more morphine. By the time they gave me the bed-bath, I'd managed to get in a couple more shots, and consequently found the all- over wash very soothing. Getting me out of bed, trailing tubes like an untidy heap of spaghetti, was more of a struggle, and once I was settled in the chair I needed a couple more shots of morphine to ease the discomfort.

But it was reassuring to be able to manage this process myself, to decide when I needed a shot of pain relief.

Every now and then the PCA machine would switch into a lunatic series of bleeps, indicating that the morphine syringe was nearly empty. The sound was truly penetrating, so it took only a few seconds for a nurse to appear and switch the alarm off. It was quite a procedure to reset it. The machine had to be unlocked, a new syringe placed inside, a series of buttons pressed in answer to questions that the machine flashed up on its little screen, and then the whole thing locked up again. If the nurse got any of the questions wrong, a torrent of abusive bleeps would erupt, and the procedure had to be repeated.

PCAs are set to prevent dangerous or too liberal use of the drugs. In my case each press of the button gave me 1 milligram of morphine. Once I had given myself one shot, the machine would ignore any requests for more until five minutes had elapsed.

I quickly learnt to control my pain, by giving myself a satisfactory background level of pain relief and more when I knew something uncomfortable was about to happen. In this way I was able to wash at the sink by the second day, and get myself out of bed unaided by the third. On the fifth day - horrors - they took away my PCA machine; on the sixth day I was moved on to a main bay of the ward, and on the eighth I was home.

Traditionally, nurse-administered analgesics, given every four hours, take little or no account of patients' differing experiences of pain. It takes 10 to 20 minutes for the drug to work, with the pain taking hold again as the effect tails off. With PCA the pain relief can be kept at a constant level with no peaks and troughs. Furthermore, there is none of the guilt-ridden inner dialogue that goes on when nurses are responsible for pain relief: is the last injection wearing off? Should I wait a little longer? Maybe the nurses are all dreadfully busy with other, iller, patients. Maybe they've gone to lunch.

There are many implications of the advent of patient-controlled pain relief. If patients are comfortable, then they are relaxed, and if they are relaxed, their bodies can concentrate on healing. In turn, the nursing staff no longer need be aware of when the next pain-relieving injection is required.

They do have to make certain that the patient is using the PCA machine to the full, though. Apparently some patients, fearful of overdosing, use the machine only sparingly. It must have been explained to me a dozen times that, with the five-minute lock-out, there was absolutely no risk of my overdosing.

With some patients the bleep is turned off, so they don't know when the machine gives them a dose and when they are locked out, with the result that the placebo effect comes into play: they think every push of the button administers pain relief, and their brains programme their bodies accordingly.

There are also impressive financial implications. My anaesthetist told me that for surgery such as mine, the use of PCA can knock two days off the average hospital stay. The cost of a bed per day on an average surgical ward is pounds 211.13 (according to the Chartered Institute of Public Finance and Accounting 1992 health database). Each PCA machine costs about pounds 1,000, while the price of the morphine used is negligible. Thus the savings on the first three patients to use each machine more than cover its costs.

The PCA machine I was using had been bought by the hospital's League of Friends.