The photo-sharing app, which has over 700 million users worldwide, appeared to be more detrimental to young people's mental health when compared to Snapchat, Facebook, Twitter and YouTube.
Chief executive of the RSPH, Shirley Cramer, said social media sites pose a real and immediate threat to the health of young people.
"Social media has been described as more addictive than cigarettes and alcohol, and is now so entrenched in the lives of young people that it is no longer possible to ignore it when talking about young people's mental health issues", Ms Cramer told The Telegraph.
Mental health crisis fueled by social media platforms created to help young people connect with others
"It's interesting to see Instagram and Snapchat ranking as the worst for mental health and wellbeing - both platforms are very image-focused and it appears they may be driving feelings of inadequacy and anxiety in young people. The platforms that are supposed to help young people connect with each other may actually be fuelling a mental health crisis", Ms Cramer said.
The report offers an insight into the possible link between mental illness and heavy social media use.
"Instagram easily makes girls and women feel as if their bodies aren't good enough as people add filters and edit their pictures in order for them to look 'perfect'", an anonymous female respondent said in the report.
"This [social media] resulted in me not eating properly and losing a lot of weight and becoming very depressed, I finally recovered which was hard for myself to be bullied online again in year 8. Overall I would say social media has caused me many issues and has caused me to be depressed many times", another respondent said.
Compare and despair
The report also identified the length of time that people are engaging with social media platforms as the greatest cause for concern.
"Seeing friends constantly on holiday or enjoying nights out can make young people feel like they are missing out while others enjoy life", it notes.
"These feelings can promote a 'compare and despair' attitude in young people. Individuals may view heavily photo-shopped, edited or staged photographs and videos and compare them to their seemingly mundane lives".
Young people who spend 2 hours plus daily connecting on social networking sites are more likely to suffer from increased levels of psychological distress, depression, anxiety and suicidal thoughts, according to the report.
However, it isn't all bad news; social media platforms can also promote a sense of community and provide emotional support for at-risk youth.
"I have anxiety and on many occasions I have found videos that put how I feel into words and explain it, and this benefits me a lot making me feel more confident", an anonymous respondent said.
Despite the criticisms levelled at Instagram by a host of respondents in the survey, the app was found to have a positive effect on self-expression and self-identity.
YouTube was the only social media platform found to have an overall positive impact on young people's mental health
In an effort to counter the negative impacts of heavy social media, experts listed a series of checks and measures designed to bolster mental health, including:
- Pop-ups warning people that they have used social media for a long time
- Social media platforms identifying users with mental health problems and "discreetly signposting places they can get support"
- Platforms highlighting when photos have been digitally manipulated - for example, fashion brands, celebrities and other advertising organisations could sign up to a voluntary code, allowing a small icon to be displayed on digitally altered photos
The impact of five social media sites were ranked as follows
- YouTube (the only platform with a positive net impact)
- Instagram (most negative)