At first glance, Slim Fast and Ensure drinks may seem similar. Both are creamy milkshake-like drinks that come in classic flavors like vanilla and chocolate. But a closer look alerts you to the fact that Slim Fast and Ensure are very different products that are used for very different purposes. By understanding the difference between caloric content, you can use both products properly when needed.

Slim Fast is used as a meal replacement diet drink in order to help you lose weight. The Slim Fast program consists of drinking a meal replacement shake for breakfast, having a sensible snack, then having another shake for lunch, another snack and finally, a healthy dinner. Slim Fast restricts your caloric intake so you see better results.

Ensure is used as a meal replacement to help raise caloric and nutrient intake when you miss a meal, need to gain weight or need extra vitamins. Ensure is fortified to act as a meal if needed, or as a healthy snack.

Slim Fast comes in a variety of flavors including chocolate, strawberry, vanilla and mocha. The Slim Fast plan comes with instructions for the diet on the packaging and the Slim Fast website offer forums and groups for support as well as healthy meal ideas for your dinner. Once you've achieved weight loss, weight maintenance includes one Slim Fast shake per day while eating a sensible diet.

Ensure also comes in strawberry, chocolate and vanilla. It is available both in cans as well as twist-off bottles that can be resealed. You may be instructed to drink Ensure when your doctor feels your diet is insufficient or you may use it when you don't have time for a healthy meal.

Slim Fast started with strictly diet drinks, but has now branched out to other products including breakfast and protein bars, notes the Diets in Review website. You can purchase Slim Fast pre-prepared in a can, or purchase it in powder form to mix yourself or add to smoothies and drinks at home.

Ensure only offers milkshakes, but the product line now offers different drinks for different health issues including bone health, muscle health and immunity.

While the various Slim Fast products have different calorie counts, a basic Optima shake contains 180 calories, which is why it's ideal for restricting your caloric intake. It's also fortified with vitamin A, vitamin C, iron and calcium, along with 10 g of protein and 5 g dietary fiber, according to PeerTrainer.com.

Ensure contains 220 calories per serving, making it ideal for those who want to gain weight or need to supplement meals. It boasts 13 g of protein yet not fiber, and also has vitamins A, C and slightly less iron and calcium than Slim Fast.

As of 2010, Slim Fast costs about $1 per shake, while other products are priced by the box. Ensure costs about $1.83 per bottle.