This comes despite the increasing popularity of weight loss drugs.
Bonny Mills, who runs the Yate Academy group, believes that lifestyle change-based weight loss techniques like Slimming World will always be more effective than drugs.
Ms Mills joined Slimming World in 1999 after attempting several methods to lose weight due to Polycystic Ovary Syndrome (PCOS), which can cause weight gain and infertility.
She lost four and a half stone in a year and naturally conceived a child.
She said: "While of course we know and our members are living proof that lasting weight loss can be achieved without drugs, we also understand that when you're desperate to lose weight, it can be tempting to try something new that promises instant results.
"The truth is though there's no magic bullet - losing weight and maintaining that weight loss requires change.
"Weight loss drugs need to go hand-in-hand with making changes to your diet, activity and mindset.
"They're not a replacement for those changes."
Slimming World focuses on helping members make lasting changes by encouraging nutritious eating and personal pace activity.
Ms Mills said: "Our healthy eating plan encourages members to eat nutritious, everyday food so they lose weight without deprivation, and we have an activity programme that helps members increase how much and how often they move, at their personal pace.
"In our groups, members get behaviour change support which helps them to understand themselves more deeply as a slimmer and change the way they think about food, activity and their weight."
She also believes that the support found in groups is indispensable for long-term change.
Ms Mills said: "Whether you're using weight loss medication or have had surgery, lasting change needs real support.
"My members form strong friendships in group, and there's nothing quite like the motivation and accountability that comes from losing weight with others who understand the challenges.
"Being part of a group helps you to stay on track and committed until the changes you're making become everyday habits."
For more details visit the Slimming World website.