But she has had it since her mid-20s and now her parents, in their 60s and unaffected by arthritis, do her heavy shopping for her.
Ms Smith was diagnosed with psoriasis in her early-20s and with psoriatic arthritis a few years later.
Like psoriasis, it is a long-term condition that can get worse with time. If it is diagnosed and treated, however, the progression can be slowed down.
Psoriatic arthritis affects several of Ms Smith’s joints, including her neck, feet and hands, as well as her back.
Taking medication, she has been able to continue her employment as a social worker, with some adjustments, but finds that her condition has impacted “every aspect of her life” since she was in her 20s.
“It affects you socially, because a lot of people with psoriatic arthritis experience chronic fatigue,” she said.
“So although I was young, I felt a lot older. If my friends were organising a weekend away or whatever, instead of looking forward to that I would be worried about where I was going to be when I was away, sleeping in a different bed, just the day-to-day things that people don't think about ordinarily, especially at that age.”
Ms Smith feels as though she has to “justify” her illness as people expect it to only appear in older people.
"People would say ‘you’re too young for arthritis’ or ‘you don't look unwell’, those kind of comments,” she said.
“You always feel like you have to justify your own illness.”
She added: “At 40 I still feel young, but there’s a lot of things … my parents do heavy shopping for me, just because doing that can make me sore the next day.
“You don't really know how it's going to affect you, so you have to pace yourself with things like cleaning your house. If I do that all in one day, the next day I can be sore.
"You have to take it into account in a lot of aspects of your daily life that other people just do without thinking.”
Ms Smith would like more people to be aware that arthritis can affect younger people.
“It is quite a common assumption that it is older people that get arthritis, people don’t understand it,” she said.
“It’s definitely seen as an older person's disease, so there’s a challenge of debunking that myth. It does affect younger people. “
The Scottish Medicines Consortium recently approved the first selective subunit inhibitor for NHS psoriatic arthritis treatment, guselkumab, made by Johnson & Johnson’s Janssen.
Professor Iain McInnes, a professor of rheumatology at Glasgow University, welcomed the approval.
“Psoriatic arthritis is a life-long disease that can have a negative impact on a person physically and mentally, and can carry a burden for the individuals with the disease, their families and society,” he said.
"At present, we do not have a sufficient number of medicines with different modes of action to manage this life-long disease effectively.”