Hearing the news that Susan Greenfield has lost her job at the Royal Institution threw me back 40 years to when she and I both went up to Oxford, to the same college and to read the same subject. This was the tail-end of the hippy era, an age of wearing wild clothes, smoking cannabis and taking LSD, listening to Grateful Dead and Pink Floyd.

We got on well but were not close friends: we were so very different. I was obsessed with investigating the paranormal and consciousness, and cared little for fame or career. She was ambitious from the start. In later years, we were often confused with each other (two Susans talking about brains on TV), although I worked at the fringes of respectable scientific topics – out-of-body experiences, memes, consciousness – without grants and usually without a job – while she went for the big time.

In some ways, she made the big time. She ended up as an Oxford professor, a baroness, a university chancellor, and director of the Royal Institution. Yet she neither did any significant scientific research nor gained the respect of most scientists. Indeed, in 2004, Greenfield was involved in another stir when several fellows of the Royal Society threatened to resign if she was elected a fellow, saying that "her work is too insubstantial and that she is too interested in self-promotion". "Self promotion" is a common accusation.

I feel sorry for my old friend and colleague, but I can only conclude that she is, in both her successes and her failures, the architect of her own fate. In her determination to get to the top, she may be an example of a woman having to fight even harder than a man to achieve such goals. So she has proved not only that you can be both a woman in chic suits and a scientist, but also that a female scientist can be just as competitive and ambitious as any man.

But what bothers me, and other scientists, is that she does not seem much to value science itself. The absolute heart of what it means to care about science is that you care about the evidence – that your opinions are based not on what you would like to be true but on what is found by research to be true.

Greenfield has, for instance, been vocal about the harms of drugs, the way they damage the brain and destroy lives. She campaigned against the reclassification of cannabis to Grade C, making meaningless comparisons with alcohol (such as that only 0.7 mg affects the brain whereas you need 2,000 mg of alcohol) – meaningless because you smoke tiny amounts of one and drink large glasses of the other. She scared people by claiming that cannabis changes who you are – but so does alcohol, so does falling in love, so does making scientific discoveries. She claimed that cannabis damages living human brain cells based on evidence from lab studies on isolated rat neurons. Worst of all, she ignored evidence on the actual harms of each drug, so painstakingly collected by Colin Blakemore, David Nutt and others.

These studies clearly showed cannabis to be less harmful than either tobacco or alcohol. We need this reliable evidence to give truthful drugs education and to create a less damaging drugs policy, but such progress is set back by Greenfield's evidence-free, high-profile pronouncements.

Then there are her dire warnings about the harms of playing computer games. This story would be funny if it were not so serious. I heard her speak last summer at the Cheltenham Science Festival, where the brochure described her "outspoken views. Praised and criticised in equal measure". There she claimed that our brains could be physically damaged by playing too many computer games. Ironically, she was simultaneously promoting her own commercial brand of brain-training device – "MindFit" – basically a simple computer game advertised as "based on scientific studies of the adaptability of the adult human brain" and "clinically proven to help you think faster, focus better and remember more". When I was recently asked to write about the evidence for brain-training games of this sort, I learned that there is no proper peer-reviewed evidence to suggest that any of them, including her own, actually improve brain function any more than playing Scrabble, chess or other computer games. And to cap it all, there is now evidence that playing fast-moving, first-person perspective computer games improves reaction times and some measures of intelligence. So she has been endorsing one unproven computer product while claiming that others do harm.

I applaud Susan for her dynamism and her many successes, but I wish she had behaved more like a real scientist.