My name is Mark Sadler. While I was studying philosophy at Reading University, it occurred to me that I could rearrange the letters in my name so that they spelled: Sam Redlark. Now some people call me Sam and others call me Mark; these days most people know me as Sam. My last steady girlfriend never knew my birth name.

I'm 29 now and will be 30 in September. I believe the things that you do in your twenties have repercussions that will carry through to the end of your life and will determine whether you struggle in your 30s and beyond. When you are young, just by virtue of your youth, there are breaks and opportunities which will in turn lead to other opportunities. As you get older, these doors begin to close until you can count the doors left open on the fingers of one hand.

When I was 19, I was in what is known as a gap year (between the end of school and the beginning of university), studying psychology at a college in London. I started to suffer from shortness of breath, just from walking up the street. My sinuses used to get clogged up and I could never hear what anyone was saying to me. I put it down to the London pollution aggravating my asthma.

The following year - now at university - I began to experience bouts of extreme nausea. Sometimes it would be so intense - like seasickness - that all I could do was lie on my bed and wait for it to stop. Studying became impossible. After a few hours of this, I would vomit partially digested food, usually into the sink in my room. The skin started to peel off my hands. On occasions, I would find my mouth suddenly full-up with blood. I would open my jaw and it would just pour out.

The doctors told me that I was depressed, then they had another think and sent me off for the usual round of tests. Eventually they concluded that there was something wrong with my liver but they didn't know what was causing it. Things were complicated further when I caught Weil's disease, probably as a result of skinny-dipping in the river Thames. One day I asked a doctor what all the orangey-brown stuff was, that I kept throwing up. He told me it was my stomach lining.

I was in my third year at university, and retaking the second year, when I read in one of the National papers about a pair of young girls who had both died from liver failure, having taken an anti acne drug called minocycline. Other people had developed severe arthritis. When I read this, I felt a mixture of relief and anger because I knew the root cause of my illness and I also knew that I could recover from it.

I dropped out of university. I tried to finish my degree but I was fighting two battles - one with my illness and one with the administration at the university. They messed me about; I can't really go into what happened because I will be libelling someone. I was so disgusted. I got into my car and left town. I never spoke to anyone at the university again. I never told them I was leaving. I just went.

What makes my three and a bit years at Reading worthwhile is the fact that I met Heinz there. He had come back from India with two nasty intestinal parasites and, like me, he was really sick. We both assumed that we were going to die anyway, so fuck it; we carried on as normally as we both could. He is without doubt my best friend - not perfect by any means - but decent and upfront. We don't see a great deal of each other anymore. We both travel a lot and he has recently moved to Greece to be with his girlfriend.

I also believe that being sick broadened my horizons. If I hadn't been ill and had graduated, I think I would have automatically settled down into a job, maybe moved in with a girlfriend; I don't think I would be very happy. If there is another possible world where that version of me exists, then I feel sorry for him. My illness made me appreciate being alive a bit more.

Directly after leaving university, I lived in my car for just over three months. I needed to get my head together. Up until that time, my whole identity had been based around academic achievement and now the rug had been pulled out from under me.

It was so cold at night and I couldn't stretch out properly, even with the back seat down. I spent Christmas day sitting on the cliffs at Lands End, drinking coffee out of a flask and listening to the foghorns of the passing ships. A huge flock of starlings landed on the grass around me. I remember looking at these birds, shaking the moisture out of their feathers, and wondering where they were going.

After a year and a bit of hopping between cash-in-hand jobs and living in squalid rented accommodation, my grandmother took me. I had some money and I decided travel to Vietnam. It was my first time outside of Europe and it was a big deal for me to go on my own and organise everything while I was out there. I only spent a month in the Vietnam but my experiences gave me the confidence to travel to other places.

I lived in Yemen for a few months and fitted in there. It's a misunderstood country. I've never met more friendly, decent people anywhere in the world. They just want a better lives for themselves and their children. The tribal skirmishes are a bit scary but if you stay out of the way, you won't get hurt. I remember one hellish taxi ride down the mountains from Sa'na to Marib, where I was suffering from the most awful food poisoning. The young Bedouin sitting next to me fell asleep and the muzzle of his Kalashnikov, which he had resting against his shoulder, kept working its way under my chin.

When I went to Yemen I was still sick and frustrated at how long it was taking me to recover. I went out there with a very nihilistic attitude and being in that country, which can be brutal and harsh, helped me work a lot of that out of my system.

In the desert outside Tarim, I shot and killed a pair of feral dogs, who were either rabid or had gone mad in the heat. Most of the time, these roaming dog packs will keep their distance, but this pair forced me to shimmy up a large shiny boulder. One of them turned around and it's entire rear flank was exposed flesh, crawling with maggots. These dogs are so dangerous because of the diseases they carry- you can't allow them to so much as scratch you. I don't believe I did anything terribly wrong in putting them out of their misery, but I still do think about it occasionally.

A few days later at a hotel in Sayun, I had what I think was a mini nervous breakdown, while I was watching the cartoon network. I think was probably the result of years of bottled-up stress.

I spent four weeks living in a Somalian shanty town on the outskirts of Al Mukala, which is a port on the South Yemen coast. As I understand it, an entire village in Somalia had decamped to Yemen and re-established itself on the cliffs outside the town.

A lot of my travelling friends started off being very adventurous and then over time, gradually settled down. I've gone the opposite route. I started off being very cautious and then, every time I went away, I got bolder and more confident. The last country I visited was Eritrea, in East Africa. I'd just ask someone where the nearest village was and then strike out across country.

I took a journalism course a few years ago. It was a good course. All but two of us went on to get jobs in the field. I left knowing that as much as I enjoyed the work, I wasn't going to be able to cut it in such a competitive profession. Around this time, my Gran, who had severe arthritis, lost her ability to walk and I took care of her until she died a year later.

Since my Gran died, I haven't really had a fixed address. I moved back in with my parents a few months ago, after I had used up the goodwill of my friends. I do not intend to be living with my parents beyond my thirtieth birthday. It's undignified. I'll move up to London and do what I have to.

I like to write short stories but I have no ambitions to be a writer. I think that ideas are unprocessed thoughts and, by developing them, you find out what you think about things and you increase your understanding. The finished writing is just the waste product of that process.

I've studied origami since 1998. It's a pure art form - all you need is a square piece of paper and some instructions and, an hour or two later, you have an insect or a tiny animal cast in paper. The detail that can be worked into a model is incredible. It's interesting that the best contemporary designers in origami aren't professional artists - they're mathematicians, engineers and scientists.

I've watched the Formula 1 racing since the late 1980s. I was heartbroken when Aryton Senna crashed and died at Imola in 1994. Just writing his name makes me feel sad. No one raced like that man. He drove without fear, as though he believed that nothing could hurt him while he was in the car. At Imola, he descended to the level of a human being again, dying in a wrecked car, in the gravel trap, while the race carried on without him.

I enjoy swimming but it's more about perfection than speed or staying fit. I want to swim perfectly.

I've been through peaks and troughs. I don't regret much. It would be foolish to try and settle down now. There's too much catching up to do, if I go down that road. If I've learned anything, it's to stop whining and just get on with living.