Care for people with autism is "varied" across England, health leaders have warned.
While there is no cure for autism, a range of treatments can help improve the symptoms, yet there is a "real variation" in the type and quality of care that people receive, t he National Institute for Health and Care Excellence (Nice) said.
Setting out a range of new standards to improve care for people with autism, Nice said that patients often have symptoms or other conditions that go undiagnosed.
It said that it is important that "clear standards" are in place to reduce variability of care.
The new guidance states that someone referred to a specialist autism team should receive a diagnostic assessment within three months.
Nice also said that drug treatments for the "core" features of autism have been shown to be "ineffective", and people with autism should not be prescribed medication to address such features.
The standards also call for people with autism who have behavioural changes to be assessed for the possible triggers behind the changes.
Professor Gillian Leng, director of health and social care at Nice, said: "People with autism can find everyday life challenging and confusing, and often have symptoms or aspects of other conditions that go undiagnosed. This quality standard outlines how to deliver the very best care and support for both adults and children with the condition."
Jonathan Green, professor of child and adolescent psychiatry at the University of Manchester, who helped to develop the new set of standards, added: "Across England, there is real variation in the type and quality of care people with autism receive, which can have lasting effects on both the person and their families or carers.
"It is important, therefore, that there are clear standards in place - based on the best available evidence and expert consensus - which specifically focus on key areas needing improvement. These will aid health and social care professionals and commissioning bodies to deliver the very best for people with autism."
Mark Lever, chief executive of The National Autistic Society, added: "The first step to getting the right support is having timely access to diagnosis, so speeding up the process will have a significant impact on the lives of thousands of people with autism in England, many of whom have waited or are waiting, to obtain this critical milestone.
"People with autism have campaigned long and hard for their needs to be addressed when professionals are designing support and services; measuring progress against this standard will help to ensure that this happens."
In England it is estimated that one in every 100 people suffer from an autistic spectrum disorder. Sufferers can have problems and difficulties with social interaction, impaired language and communication skills and unusual patterns of thought and physical behaviour.