Some high street cosmetic clinics exaggerate benefits and downplay risks, an investigation has suggested.
Botox was one of the treatments sought
Which? magazine also found illegal advertising, and one doctor under investigation for serious professional misconduct.
Undercover researchers visited 16 clinics for advice on treatments such as botox injections, laser treatments and chemical peels.
But the clinics criticised have rejected the Which? findings.
Four researchers sought advice at 16 clinics in London on botox treatment to eradicate lines, collagen injections for lips, chemical peels and laser treatment for facial lines.
The consultations were recorded and assessed by a consultant plastic surgeon and two nurse specialists.
Half were classed as poor - and only five were thought to be "good".
The panel highlighted claims made by Dr Fayez Abu Mahfouz at the London Cosmetic Laser Centre, who told the Which? researcher: "Almost anything you want, I can do it, and that's a guarantee, it's fact, not fiction."
Dr Mahfouz is currently being investigated for misconduct following patients' complaints.
Potential problems with botox
Paralysis of the muscles
Lopsided facial features
But Tony Child, solicitor for the clinic and Dr Mahfouz, told BBC News Online: "The clinic operates a policy of a free initial consultation - which is obviously what's taken place here.
"If the condition is susceptible to treatment, the next stage would be a consultation where Dr Mahfouz would go through the patient's medical history and how successful the treatment is likely to be.
"The Which researcher obviously didn't go on to this stage."
He said the "guarantee" referred to results. "We will achieve what we say we will achieve - or you get your money back."
Mr Child added that the complaints against Dr Mahfouz were "unfounded and misplaced".
At the Yannis Alexandrides clinic in London, the researcher waited for 50 minutes before being charged £100 for a five-minute discussion.
She was told Botox, a prescription-only drug which can have serious side effects, was just a "quick, painless, simple injection. You don't need a consultation".
But Dr Alexandrides said: "I am surprised that they
found the consultation was only five minutes because the discussion is normally up to 40 minutes.
"If it was five minutes it must have been that they did not have any questions to ask."
He said he always explained potential side effects from treatment to prospective clients.
Prescription-only medicines should not be advertised directly to consumers, but Which? found the National Cosmetic Centres in London sent out invitations to a "Botox Party".
Which? said it had reported the invitation to the Medicines and Healthcare products Regulatory Agency (MHRA).
But Robert Houtman, director of the centres, said: "This was not an advertisement. It did not appear in the press.
"It was an invitation which was sent to key people and was available in our clinics. It was never in the public domain."
Only consultations given at Manchester's Heaven Spa and the Harley Medical Group in Manchester and London, the Hurlingham Clinic and Spa in London and Liinzi James Clinic in London were considered to have taken good medical histories and provided thorough details about risks and side effects.
Malcolm Coles, editor of Which?, said: "The hype surrounding high street cosmetic procedures suggests anybody can pop out on their lunch hour and return to work looking years younger.
"But our research uncovers a worrying lack of safety checks and shows that providers are glossing over risks.
"Inadequate and inappropriately performed cosmetic treatments can lead to facial scarring, blistering, droopy eyebrows or a trout pout.
"If you're thinking of taking this step, you need to thoroughly research the clinic, practitioner and treatment - and find out about the risks as well."
Simon Withey, a consultant plastic surgeon working in London, agreed that there was a problem.
He said: "Many patients come to you saying they have not been consulted properly, they don't know anything about the products they are being given, and it really is of great concern."
Dr Patrick Bowler, head of the British Association of Cosmetic Doctors, told BBC News Online patients could check if their doctor or nurse was registered with an association such as the BACD.
He said: "A personal recommendation is probably one of the best ways to find a clinic.
"But if you have a gut feeling something's not right, that a doctor is being too pushy, say you need time to think about it.
"It's generally thought that a cooling-off period is a desirable thing."