Optislim and Optifast are the two names almost everyone runs into when they start looking at meal replacement shakes in Australia. They sound similar, they sit on the same pharmacy shelf, and both promise fast weight loss. They are not the same thing, they are not owned by the same company, and the one that suits you depends mostly on whether you want a doctor involved. This guide compares them properly: what’s actually in the shakes, what each costs per meal, what the research does and doesn’t show, the side effects nobody mentions in the ads, and who each one genuinely suits.
Optislim vs Optifast: the Short Answer
| Choose Optifast if | You want a medically supervised program, you have a health condition (type 2 diabetes, heart disease, high blood pressure), you’re preparing for surgery, or you want the brand with the deepest research record |
| Choose Optislim if | You have less weight to lose, you’re generally healthy, you want to replace one or two meals rather than all of them, you want it cheaper, and you’re happy managing it yourself |
| Either way | A full very-low-calorie diet (all meals replaced) is a medical intervention, not a diet trend. Talk to your GP or pharmacist first, whichever brand you pick |
What a VLCD Actually Is
Both brands sell what the research literature calls a formula total diet replacement. Instead of cooking, you drink nutritionally complete shakes (or eat bars and soups) that between them supply roughly 800 calories a day, along with the full daily requirement of protein, vitamins and minerals. Below about 800 calories a day it’s a very-low-calorie diet (VLCD); a bit above that it’s a low-calorie diet (LCD).
The reason this works isn’t magic: it’s a large, enforced calorie deficit, with the protein and micronutrients kept intact so you lose fat rather than muscle and don’t become deficient. After a few days your body shifts into mild ketosis, which blunts hunger, most people find days three to five are the hard part and it gets easier after that.
Optislim: What It Is
- Who owns it: an Australian company, on the market since 2001 and made in Australia.
- How you buy it: direct from optislim.com.au and in supermarkets and pharmacies. No referral, no appointment, no supervision required.
- The range: around 47 products. VLCD Classic and VLCD Platinum shakes, bars, soups, low-calorie snacks, plant-based options, separate “for Her” and “for Him” lines, and a newer range aimed at people on GLP-1 medications like Ozempic.
- The program: self-guided, in three phases, rapid weight loss, steady weight loss, then maintenance. You choose whether to replace one, two or three meals a day.
- A VLCD Classic shake (43 g sachet): 692 kJ / 165 calories, 17.7 g protein, 3.4 g fat, 17.4 g carbohydrate (13.7 g sugars), 0.8 g fibre, plus added vitamins and minerals. Contains milk; the Classic chocolate also contains wheat protein (gluten) and soy.
Optifast: What It Is
- Who owns it: Nestlé Health Science. Optifast has been used internationally for over 50 years and is sold in more than 30 countries.
- How you buy it: mainly through pharmacies, and the company’s own labelling states it “must be used under the supervision of a healthcare professional”. It’s positioned as a clinical product, not a supermarket one.
- The range: shakes in six flavours, ProteinPlus shakes, soups, bars, desserts.
- The program: four structured levels. Intensive (all three meals replaced), Active 2, Active 1 (fewer replacements as food returns), and Maintenance, with an app, recipes and healthcare-professional support.
- A VLCD shake (per serve): 850 kJ / 203 calories, 20 g protein, 4.6 g fat, 18.5 g carbohydrate (12.7 g sugars), 3.6 g fibre, 400 mg calcium and a full vitamin and mineral profile. Milk-based and gluten free.
Side-by-Side Comparison
| Feature | Optislim | Optifast |
|---|---|---|
| Owner | Australian, est. 2001 | Nestlé Health Science |
| Sold through | Own website, supermarkets, pharmacies | Pharmacies, under healthcare supervision |
| Medical supervision | Optional | Expected, stated on the packaging |
| Program structure | 3 phases, self-guided | 4 levels, structured, app-supported |
| Calories per shake | 165 cal (43 g serve) | 203 cal per serve |
| Protein per shake | 17.7 g | 20 g |
| Fibre per shake | 0.8 g | 3.6 g |
| Calcium per shake | 279 mg | 400 mg |
| Gluten | Some products contain wheat protein | Gluten free |
| Typical cost | About $2.10 to $2.25 a meal (21-meal pack around $45) | Higher; pharmacy pricing, typically $3 to $4 a meal |
| Research record | Marketing cites third-party VLCD research (see below) | Long publication record; the brand cites 80+ scientific publications |
| Availability | Australia | 30+ countries |
Nutrition figures taken from each brand’s own current product labelling (checked September 2026); prices change constantly, so treat them as a guide rather than a quote.
What the Research Actually Says
This is where the two brands genuinely differ, and where the marketing on both sides needs a second look.
Formula diets do work: that part isn’t in doubt
The evidence for total diet replacement as a category is strong. In the landmark DiRECT trial, published in The Lancet in 2018, people with type 2 diabetes who followed a formula diet of 825 to 853 calories a day for three to five months lost an average of 10 kg, and 46% were in diabetes remission at one year, compared with 4% on usual care. At two years, more than a third were still in remission. The DROPLET trial in the BMJ found much the same in general practice: people referred to a total diet replacement programme lost 10.7 kg at one year versus 3.1 kg with usual care.
A 2017 systematic review in the International Journal of Obesity found that formula diets produce about 0.5 to 0.6 kg of weight loss per week, and, usefully, that people with type 2 diabetes lose just as much as people without it, contrary to the old assumption that diabetes makes weight loss harder.
But “clinically proven” doesn’t always mean what you think
Almost none of that research tested these specific brands. Optislim’s product pages describe “our 12-week clinical study, based on 1,200+ participants” and cite a PubMed ID. We looked the reference up: PMID 25586630 is Rolland and colleagues, published in Clinical Obesity in 2013, a retrospective analysis of 710 people following a 550-calorie community VLCD programme with group support and behaviour therapy. It’s a real and reasonable piece of research about VLCDs in general, but it isn’t a 1,200-person trial of Optislim’s own shakes.
Optifast is on firmer ground: it appears by name in published clinical research, is used in hospital and pre-surgical weight-loss protocols, and the wider evidence base for its style of medically supervised program is the one that produced results like DiRECT. That’s the practical difference between the two, not that one works and the other doesn’t, but that one has been formally studied and the other leans on the category’s research.
Worth saying plainly: both brands are nutritionally complete formula products. A calorie deficit is a calorie deficit. If you follow either properly, you will lose weight.
Cost: What You’ll Actually Spend
On a full VLCD you’re replacing three meals a day, so the per-meal price matters more than the pack price.
- Optislim: a 21-meal VLCD Classic pack is around $45, roughly $2.14 a meal, about $45 a week on three replacements a day, with multi-pack discounts and free shipping over $99.
- Optifast: sold by the box through pharmacies at a higher per-serve price, typically in the $3 to $4 range depending on the product and any promotion, so roughly $65 to $85 a week on three replacements, before any consultation fees.
- Don’t forget the extras: both programs expect you to add low-starch vegetables, and Optifast’s structure may involve GP or dietitian appointments (some of which may attract a Medicare rebate under a chronic disease management plan, ask your GP).
- The honest comparison: against a typical grocery and takeaway spend, a VLCD isn’t necessarily more expensive. It just moves the money.
Taste and Day-to-Day Reality
“What tastes better, Optifast or Optislim?” is one of the most searched questions about these two, and the honest answer is that it’s personal, but a few patterns come up consistently in user reviews and pharmacy feedback:
- Optislim is generally described as sweeter, with more flavour variety (chocolate, vanilla, strawberry, banana, mango, choc mint, coffee, salted caramel).
- Optifast is described as more “clinical” tasting and thinner, though the ProteinPlus and dessert formats give you more texture to work with.
- Both are far more tolerable made with icy-cold water, blended rather than shaken, and left to sit for a few minutes.
- Flavour fatigue is the real enemy on a full VLCD. Variety packs and mixing formats (shake, soup, bar) matters more than which brand you start with.
Side Effects and Safety
These apply to both brands, because they’re features of very-low-calorie dieting rather than of any particular shake:
- The first week is unpleasant. Headache, fatigue, irritability, feeling cold and bad breath are normal as you shift into ketosis, and usually settle by day five.
- Constipation is very common. This is where Optifast’s 3.6 g of fibre per serve has a practical edge over Optislim Classic’s 0.8 g. Add the permitted vegetables, drink at least two litres of water a day, and ask a pharmacist about a fibre supplement.
- Gallstones. Rapid weight loss increases the risk of gallstone formation. It’s one of the main reasons VLCDs are meant to be supervised and time-limited.
- Muscle loss is minimised but not eliminated by the high protein content. Keep doing some resistance exercise.
- Medication changes. If you take insulin, sulfonylureas or blood pressure medication, doses often need adjusting quickly once you start, this must be managed by your doctor, not by you.
- Not suitable in pregnancy or breastfeeding, for children and teenagers, for people with a history of eating disorders, or for anyone with significant kidney, liver or heart disease unless a specialist says otherwise.
- Regaining weight is the usual outcome if the maintenance phase is skipped. Both programs include one for good reason: the shakes are the easy part, and the transition back to food is where the result is won or lost.
Which Should You Choose?
- Lots of weight to lose, or a health condition in the mix: Optifast. Supervision isn’t red tape here, it’s medication adjustment, monitoring and someone checking the maintenance phase actually happens.
- Pre-surgery weight loss: Optifast, because that’s the protocol most Australian surgical teams already use.
- 5 to 15 kg to lose and otherwise healthy: Optislim, replacing one or two meals a day rather than all three, is cheaper and easier to live with.
- You struggle with constipation or need gluten-free: Optifast’s higher fibre and gluten-free formulation is the easier fit.
- Budget is the deciding factor: Optislim, and put the difference towards the maintenance phase, where most people fall down.
- You’re on a GLP-1 medication (Ozempic, Wegovy, Mounjaro): both brands now market nutrition support ranges for this. The concern with GLP-1s is protein and micronutrient intake while appetite is suppressed, that’s a conversation for your prescriber, not a reason to add a VLCD on top.
Frequently Asked Questions
Is Optislim the same as Optifast?
No. They’re competing brands from different companies. Optislim is Australian and independent, Optifast belongs to Nestlé Health Science. The similar names and similar products cause a lot of confusion, but they’re separate ranges with different formulations, prices and distribution.
Which is better for weight loss, Optislim or Optifast?
For pure kilos lost, neither has a proven advantage, weight loss on a VLCD is driven by the calorie deficit, and both supply roughly the same. Optifast has the stronger clinical track record and the supervision that makes a full very-low-calorie diet safer, so it’s the better choice for larger or medically complex weight loss. Optislim is the better everyday option for smaller, self-managed goals.
What’s the difference between Optifast and Optislim shakes?
Per serve, Optifast has more calories (203 vs 165), more protein (20 g vs 17.7 g), considerably more fibre (3.6 g vs 0.8 g) and more calcium (400 mg vs 279 mg), and is gluten free. Optislim is cheaper per meal, comes in more flavours and is easier to buy without a pharmacy visit.
What tastes better, Optifast or Optislim?
Most people find Optislim sweeter and more dessert-like, and Optifast plainer. Taste is the single biggest reason people abandon these programs, so if you can, buy a variety pack or a couple of single sachets before committing to a month’s supply.
How much weight can you lose on Optislim or Optifast?
Research on formula diets shows roughly 0.5 to 1 kg a week on average, with trials of full total diet replacement producing around 10 kg over three to five months. Both brands advertise faster figures; the first week always looks dramatic because much of it is water and glycogen.
Can you buy Optifast without seeing a doctor?
It’s sold in pharmacies and the pharmacist can advise you, but the labelling states it should be used under healthcare supervision. For a full three-meal-replacement VLCD, that supervision matters, particularly if you take any regular medication.
Can you buy Optislim or Optifast outside Australia?
Optifast is sold in more than 30 countries, though the formulation and program differ by market (in the US it’s typically offered through clinic-based programs). Optislim is an Australian brand and sold mainly in Australia.
How long can you stay on a VLCD?
Total meal replacement is usually limited to around 12 weeks, after which you step down through the program’s transition phases. Staying on one indefinitely without supervision isn’t safe or necessary.
The Bottom Line
Optifast is the clinical option: more research behind it, more fibre and protein per serve, a more structured program, more expensive, and designed to be run with a healthcare professional. Optislim is the accessible option: cheaper, tastier to most people, easy to buy, flexible about how many meals you replace, and fine for a healthy adult with a moderate goal.
The thing that determines whether either works isn’t the brand on the sachet. It’s whether you complete the maintenance phase and change what you eat afterwards. Choose the one you can actually stick to, and plan the way back to real food before you start.
Related Reading
- Natural Ozempic Alternatives: what the evidence says about the supplements people try first
- Diet and Nutrition: all our nutrition guides
- Lifestyle and Wellness: sleep, stress and everyday habits
This article is for educational purposes only and is not medical advice, and we have no commercial relationship with either brand. Very-low-calorie diets are a medical intervention: speak to your GP, pharmacist or dietitian before starting one, especially if you take regular medication, are pregnant or breastfeeding, or have diabetes, kidney, liver or heart disease. Nutrition and price details were taken from each brand’s own published information in September 2026 and may change. See our medical disclaimer.