THE Hippocratic Oath commits doctors to using their knowledge to help the sick and do no harm. It also pledges them to respecting patient confidentiality, stating that “whatsoever in the course of practice you see or hear that ought never to be published abroad, you will not divulge”.

“As a doctor who is also a writer,” writes Gavin Francis, “I've spent a great deal of time deliberating over that use of 'ought', considering what can and cannot be said without betraying the confidence of my patients.”

This dilemma is aired in an author's note to Francis's new book, a meditation on medicine and human change that draws heavily on his own caseload, and the author's solution is to ensure that all those mentioned are “so disguised as to be unrecognisable”. They are, however, vividly described. In a chapter exploring the physiology and mythology of conception we meet Hannah, a Welsh-accented hippy with multicoloured hair, a fondness for long purple skirts and three unplanned pregnancies. In a section on tattooing we encounter Mark, a thin, fair-haired heroin addict in his early 40s with “tight, bloodless lips”.

Assuming those pen portraits are invented (I'm visualising the real “Hannah” as a conservative dresser from the Home Counties and “Mark” as a dark, heavy-set 20-something with a generous mouth), we might wonder what they are for, though Francis – a multi-award-winning author of several books – clearly knows that the art of good storytelling lies is in the detail, and there is no doubting the authenticity of the human experiences he describes.

Indeed, the insight he offers into the physician's realm is profoundly affecting. Francis, a Fife-born Edinburgh-based GP who previously worked as a doctor in Antarctica and West Africa, comes across as an extremely thoughtful, deeply humane practitioner. Drawing on classical literature and medical history, as well as his own clinical experience, Shapeshifters provides a fascinating account of humankind's efforts down the millennia to understand our minds and bodies in order to prolong life and wellbeing.

“To be alive,” writes Francis, “is to be in perpetual metamorphosis,” and references to Ovid's Metamorphoses recur throughout the book. “Everything is in a state of flux,” declared the Latin poet and Francis is drawn to the same subject because, he writes, “to practise medicine is to seek positive change, however modest, in the minds and bodies of my patients”.

The theme of change is interpreted very broadly in Shapeshifters. There are chapters on life's natural progressions, such as pregnancy, puberty, menopause and death, and on humankind's more ostentatious attempts to influence the raw material we are born with, through bodybuilding, cosmetic surgery or the use of hallucinogenic drugs.

There are encounters with individuals who'd once have been persecuted as demons. Francis witnessed a patient who was growing a horn on her forehead, and he helped treat someone whose “agitated delirium” turned out to be acute porphyria, a rare condition that can cause upper-facial hair growth, acute sensitivity to sunlight, a tendency to howl with pain and – in the bad old days – the label of werewolf.

In one of the most moving chapters, Francis conveys the wonder of delivering his first baby then recalls diagnosing a tiny infant with a heart murmur: a condition which, until the 1930s, often prevented sufferers from reaching adulthood. In the 1930s, pioneering American surgeon Robert Gross, “convinced that this devastating problem had a simple, technical solution”, began experimenting with ways of closing off the faulty heart vessel concerned, working on cadavers, perfecting his technique on anaesthetised dogs and finally, successfully attempting the procedure on a child. His breakthrough transformed paediatric surgery and the life chances of generations of babies and children.

Surgical transformations of a different kind are explored in a highly topical section on gender identity, which reminds us that Greek literature is replete with stories about people changing sex and suggests that the 20th century's more rigid attitudes were partly a product of “the hardening rationality of the Enlightenment”. Classed as “sexual deviation” by the American Psychiatric Association in the 1950s, what we now term “gender variance” was later depicted as “gender identity disorder” and “gender dysphoria”. And while the latter term is disliked by those who are entirely comfortable in their adopted gender, Francis's young patient “Tarik” was, we are told, profoundly unhappy with his masculine physique until, following a course of prescription drugs and surgery, he became Teresa and “her dysphoria was replaced by euphoria”. Clearly, however, the surgical techniques involved in transforming genitalia are not for the faint-hearted and Francis makes a subtle plea for a more relaxed approach towards androgyny and gender ambiguity in the hope, perhaps, that people's choices needn't always be “quite so stark”.

“Everything is in flux,” the book's final sentence reiterates - which is so obviously true that I do wonder if transition is almost too broad a theme, allowing coverage of a huge array of subjects from gigantism to jetlag, and the shoehorning in of a couple of previously published sections including a gruesome account of a pathologist's working day. (“I only wear a mask when there are maggots,” she tells Francis as she slices up cadavers.)

It's fascinating territory, however, and Shapeshifters is beautifully written as well as extremely absorbing. I came away from it with a renewed appreciation for the wonders of the human body, as well as an enhanced respect for the men, women and gender-variant individuals who spend their lives trying to make ours better.

Shapeshifters: On Medicine And Human Change

Gavin Francis

Wellcome Collection/Profile Books, £16.99