ADHD was first recognised in adults in 2008, eight years after childhood ADHD was formally recognised through National Institute for Health and Care Excellence (NICE) in most of the UK NHS (England, Wales, and NI).
The DoH said whilst a level of provision is available, it is aware that "this has also led to inconsistencies between trusts".
Sarah Salters, CEO of ADD-NI, said the charity receives a high number of weekly referrals for ADHD support programmes.
Since January they have had 165 referrals, with almost half of those for adults.
She said the lack of services within the health service for adults is "diabolical", leaving adults "very frustrated" with "nowhere to turn".
Ms Salters said many people are "forced" to go private for a diagnosis and also have difficulty getting medication as not all GPs honour the shared care protocol.
"About 4% of our adult population have ADHD, that's about 70,000 people across Northern Ireland and there's possibly a very high number of people who are undiagnosed," she said.
"Where you get the right support, the proper strategies in place, people with ADHD start to see that they can do things that they weren't capable of doing before."