Shortly before her 30th birthday, Ellen Forney was diagnosed with manic depression. Her pattern was fairly normal. "Throughout my 20s I had moderately extended periods of a more or less good mood and moderately extended periods of more or less down moods that became increasingly distinct from one another," she says. "But I didn't really progress to a state that could be considered an episode or a disorder until I was about 28."

It took Forney over a year to admit she had a problem and get help. It took four years for her to find the right cocktail of medication to keep her mood swings in check. That was more than 10 years ago. Now Forney, an illustrator and cartoonist from Seattle, has published a graphic memoir of her mental illness.

Her book, Marbles: Mania, Depression, Michelangelo & Me, is an unflinching and frequently unforgiving narrative of what it means to have bipolar disorder and how treatment can often seem more terrifying than the illness itself. Though, as Forney admits: "By definition I was mentally ill, so perhaps I wasn't always the most rational of judges."

Forney had the idea for the book for some time, but held off writing and drawing until she was sure that she could cope with the consequences. "Like most people with a mental illness, I'm only too aware of the fragility of my emotional state," she says. "Even when I'm feeling OK, I  have a nagging sense I might be on borrowed time. That something might change. So I didn't want to start work on something so sensitive and personal until I'd had some time to trust my recovery. I needed to believe that revisiting the most acute phases of my illness wouldn't be too traumatic, and also that I could cope with the reception – good or bad – of writing about it. When you go public and talk about something openly, you have no control over how people may react."

Fortunately, the response in the US, where her book was first published, has been overwhelmingly positive. The  combination of words and drawings turned out to be a powerful medium for exploring manic depression, with cartoons representing the emotional extremes which words seldom seem to do justice to.

"The depressions were certainly more painful for me to experience and for friends and loved ones to witness," Forney says. "But, curiously, looking back, the manic episodes now feel rather more dangerous.

"The euphoric parts were amazing. Colours felt vivid and vibrant; the world felt fascinating and interconnected – and I felt powerful, sexy, and full of love and curiosity. But I was also insatiable, impatient, compulsive and restless. I cringe now at how offputting to other people I might have been."

For Forney's friends and family, the depressive episodes were the easiest to cope with. She never at any time felt suicidal, nor did she exhibit any such tendencies. "I don't take any real credit for this," she says. "It wasn't some great show of inner strength. I just think I was lucky. Suicide never felt like a serious option; when I was depressed, my main focus was just on trying to get well."

Finding the right medication to stabilise her moods was a long and frustrating period. Over four years, Forney's psychiatrist, Karen, who features as one of the key characters in Marbles, tried her patient on different doses of various drugs, including lithium, clonazepam and lamotrigine; some had little or no effect, some had unpleasant side-effects, such as bringing her out in acne or lowering her blood count. Others were just too expensive in the US, where there is no NHS or standardised prescription charge. Eventually the right dosage of lithium was found.

So, did Forney ever wonder if the medicines were making any real difference to her condition? "Of course," she says. "I think every patient with a mental illness wonders that at some point. But stopping a drug regime that appeared to be working, even if it was only coincidentally, was not a chance that I felt I could afford to take. The possibility of relapsing into manic depressive mood swings was unbearable.

Still, for Forney, facing up to the fact that she needed to remain on medication was not easy. "I was worried that my creativity would be lost," she says. "That the price of having my bipolarity under control would in some way make me not me. I still held on to the romantic notion of the artist as some kind of mad genius."

Forney also attempted to disguise another significant aspect of her life from her psychiatrist. "I lied to Karen for several years about how much dope I was smoking," she says. "I told her I was only getting stoned a couple of times a week, when it was actually more like five days a week.

"Eventually, I figured there might be some connection between my dope smoking and the severity of my mood swings. So I confessed and decided to quit taking drugs. I wouldn't say that everyone with manic depression needs to stop smoking dope, but it has worked for me. I don't feel any less creative than I was before. And if I am, there's more than enough compensation in being stable and productive."