A support group for an incurable neurological condition said a "perfect storm is brewing" over "mass cancellations" of appointments.
Dystonia causes uncontrollable muscle spasms and is commonly treated with Botox injections which paralyse nerve connections.
They are described as a "lifeline" to people who would otherwise be left in chronic pain.
But patients are having treatments delayed at Wales' largest hospital.
Cardiff and Vale university health board apologised saying it has been "challenging to deliver the service" at the University of Wales (UHW), Cardiff.
What is dystonia?
- Dystonia is a movement disorder that can cause muscle spasms and contractions
- Movements are often repetitive and can cause unusual, awkward and painful postures
- It is thought to be caused by a problem with the part of the brain that controls muscle movement
- There is no cure but can usually be managed through medication, physiotherapy or surgery
- It is estimated at least 70,000 people are affected by dystonia in the UK, including 8,000 children and young people
Graham Findlay, 60, from Cardiff, founded the support group Dystonia Cymru 10 years ago, having had the condition all of his life.
He described it as his brain sending the "wrong signals" to the muscles in his neck and shoulders which causes spasms and affects his speech.
Botox has to be injected every two or three months as the nerve cells re-grow, and "if there's a gap, the symptoms rapidly return".
"They can be very distressing and painful to patients," he said.
Mr Findlay estimates about 300 people due to attend the Botox clinic at UHW have received cancellation letters "due to unforeseen circumstances".
"This has understandably created anxiety amongst our members," he said.
He said there had been a history of cancelled appointments over the last four years, but matters "have now reached a situation that can only be described as a perfect storm".
He said it was down to the amount of referrals, the fact that patients "don't drop off the list once referred", the lack of qualified injectors and staff absence.
Cardiff and Vale university health board said the UHW clinic is the only dedicated clinic for dystonia patients in south east Wales.
A spokeswoman said: "Unfortunately, over the last six months, due to the increased demand and staff shortages it has been challenging to deliver this service."
"With the new clinic space available to us and the opportunity for training more nurses and doctors to assess and treat patients with dystonia, we hope to resume a normal service in the near future."
Mr Findlay has met with the health board's chairwoman, Maria Battle, and has also written to Health Secretary Vaughan Gething.
"We feel we have been offered a Cinderella service for years and the time is due for root and branch reform," he said.
"We want our specific condition to be taken seriously."
A Welsh Government spokesman said: "There are ongoing discussions with Cardiff and Vale university health board, the main centre for Botox clinics, and the other health boards in south Wales, in regards to opening up more clinics," he added.