More than 8,000 people came forward to have their drugs tested at music festivals in the UK this summer.
Seven festivals in England have had "front of house" services provided by The Loop at which festival-goers can have their drugs tested anonymously.
Testers found drugs four-times stronger than normal or being mixed with anti-malaria tablets and plaster.
Drug testing aims to give people information about what they are taking but critics say it "could mislead".
The main drugs checked by The Loop, a not-for-profit organisation, were MDMA, ketamine and cocaine.
The chemists also detected a growing use of N-ethyl Pentylone, which was increasingly mis-sold as MDMA and could lead to psychosis or up to three days of insomnia.
Public testing by The Loop
These are the seven festivals the organisation provided "front of house" testing facilities, where the public could get their drugs tested.
- Bestival, Dorset
- Boomtown Fair, Hampshire
- Love Saves the Day, Bristol
- Y-Not Festival, Derbyshire
- Boardmasters, Cornwall
- Kendal Calling, Cumbria
- MADE.Festival, Birmingham
Behind the scenes testing was also carried out at Parklife, Manchester; Truck Festival, Oxfordshire; South West Four, London; Lost Village, Lincolnshire and Mistress Mary, Cambridgeshire.
The tests also detected a wide range of substances used to cut other drugs, ranging from crushed anti-malarial tablets and plaster of Paris to body-building supplements and sugar.
Guy Jones, senior chemist for The Loop, said: "About half [of people having drugs checked] say that they will take smaller quantities after speaking to our healthcare professionals about strength and dosage."
He added that one-in-10 festival goers handed over more substances for disposal by the police after having drugs tested.
The Loop is compiling the information it gets from festivals for research it is planning to present to the Home Office and is aiming to attend more festivals in 2019.
Founder Fiona Measham, professor of criminology at Durham University, said: "It's a fine balance between letting people know what is out there and not being seen to encourage drug use."
However, not every festival will allow The Loop onsite to carry out drug tests and not everyone is convinced drug testing is the right thing to do.
The Loop allowed the BBC to see their operation at the Boomtown Festival in Hampshire as part of an investigation into drug use at music festivals. Viewers in the South of England can watch more on Inside Out at 19.30 BST on Monday, September 3 on BBC One.