Antidepressants saved my life, I am sure of that. But I am also certain they made my mental illness much worse too. It has taken just under two years from my first very dark thoughts to me feeling sane and — largely — back in control of my mind. That’s not merely because it takes time to heal, but because it took at least six months for the doctors to work out what pills to give me.

My symptoms of anxiety and depression started in the spring of 2016, and the first few drugs I was prescribed didn’t work. In fact, they really did just make things worse. The GP listened to me describing my symptoms, and handed me a course of Citalopram, a very common antidepressant. Counselling would be essential too, she said, but the local NHS was so overstretched that it could take a year before I’d get any.

I started having private counselling and waited for the drugs to work. They didn’t. In fact, I realised that I was becoming more paranoid and agitated. I grew convinced that I was going to be sacked from work, that my friends hated me, and that my new partner was on the brink of leaving me. The constant feeling of panic was exhausting, and to top this off, I couldn’t sleep. The doctor doubled my dose and told me to keep going: sometimes these things took a while to work. By the early autumn, I was struggling to make it through a couple of days — let alone a whole week — of work, and spent most evenings in a blind panic about my personal life. I still couldn’t sleep.

The doctor suggested adding another drug, Mirtazapine, which would calm me down. This medicine did what it promised, but it calmed me down so much that not only did it take me an hour before I could even crawl to the bathroom in the morning, it also slowed down my metabolism and I gained a stone and a half in a month. I had been the same size and weight since I was 21, but now I was swelling like a marrow.

And still I wasn’t improving. In fact, by this point I had become too ill to work, breaking down totally at the Conservative party conference and needing emergency sedation. I went back to the doctor again. ‘I’ve never worried all that much about how I look,’ I said. ‘But if I keep on taking this drug I might start. Plus I’m now off work and I’m frightened that I’m never going to be able to go back.’

The doctor weaned me off Citalopram and Mirtazapine, and tried another common drug, Sertraline. I was referred to a psychiatrist (privately, as the NHS waiting list was unbearably long), who considered diagnoses other than the common (and rather vaguely defined) anxiety and depression. Over the following months, the symptoms of what now appeared to be post--traumatic stress disorder started to improve a little.

But it was only after the doctors tripled my dose of Sertraline and prescribed yet another pill to take in the evenings; a sleeping pill with an antidepressant effect called Trazodone, that I regained my sanity. Those pills have their side-effects, too, but I was never forewarned about the weight gain, night sweats or other unpleasant problems antidepressants can cause. I just learned about them when they turned up uninvited.

I’m not even sure the medication would have worked without my therapy and the Olympic-style regime I’ve set up to keep my mind as fit as possible. I ran so much last year that my foot packed in. Now I cycle obsessively and go for a daily short walk to calm my mind. I try to redirect my paranoia into obsessing about nature, enrolling on a distance-learning botany course as well as taking up birdwatching. When I told an MP about my new obsession with kingfishers, he told me: ‘Isabel, I never thought I’d say this to anyone in Westminster. But you really need to get out less.’

Does it really have to take so long to recover? My experience is typical: Citalopram and Sertraline are ‘first-line’ drugs that doctors try out on patients with depressive symptoms. When those don’t work after a few months they move on to others. But no one knows which drugs work best for which patient. There is so little research that much treatment is pure trial and error.

But the time, and health, lost during those months of trial and error comes at a huge cost to the patient and to wider society. A recent government report on mental health and the workplace estimated that the economy loses between £74 billion and £99 billion a year as a result of mental illness, with employers losing up to £42 billion from staff ill-health.

Nothing like those sums is spent on improving the treatment of depression. In fact, mental health represents only 5.8 per cent of the total UK health research spend: £8 per person affected by mental illness. That figure is 22 times higher for cancer.

No wonder cancer treatment has made such thrilling advances in recent years. The prognosis for many cancers is dramatically better as a result of the vast sums of money that have rightly been spent researching treatments for this killer illness. But mental health needs to catch up. Many of its treatments are from another era and the basic design of antidepressants, anti-anxiety medication and antipsychotics has barely changed since the 1950s. The newest anti-depressants are no more effective than the first modern one, which was called Imipramine. Science has moved on in so many areas, but not when it comes to mental illness.

The charity MQ is funding work on how to predict which pills will work best for which patient. Led by Dr Claire Gillan at Trinity College Dublin, this will develop an algorithm to estimate how well someone with certain symptoms will respond to a particular medicine. It could mean that trial and error is replaced by something more akin to cancer treatment, where drugs are developed to work well for specific groups of patients. Herceptin, for instance, is only given to patients who have tested positive for HER2 cancer. Currently, we don’t know enough about what lies behind the many different types of depression to tell which drugs will help.

I’ve recently started reducing my dose of Sertraline. I could, if I needed to, live with the side-effects for life, but I’d rather not. I do, though, look back over the past two years and feel a pang of sadness, not so much for how hard everything has been, but for how unnecessarily long it took to start the right treatment, let alone recover.