Patient Suitability


The use of a VLCD may influence the dosage requirements of some medications. This is important for medications that have a narrow effective therapeutic range. The mechanisms that may alter requirements include:

  • A major change in the nature of dietary intake – macronutrient and micronutrient
  • Significant negative energy balance and resultant rapid weight loss
  • Ketosis associated with fat catabolism
  • Alterations in body composition with weight loss: – Reduced fat/lipid compartment – Altered lean body mass – Altered hydration.

Individuals receiving medication for Type 1 & Type 2 diabetes, hypertension, dyslipidaemia or those on lithium therapy may need a reduction in dose or withdrawal from their medication whilst undergoing a VLCD program. Such individuals should be monitored carefully in the first few weeks of using a VLCD.

No Interactions

The following list of medications can be used normally in patients on a VLCD:

  • Minor tranquilizers
  • Antibiotics
  • Anti-emetics
  • Anti-diarrhoea agents
  • Antacids
  • Oral contraceptives
  • Oestrogen for the prevention of osteoporosis
  • Antihistamines


The following list describes the situations in which medications should be decreased or special care is needed.

Insulin or oral hypoglycaemic agents for Type 1 & Type 2 Diabetes

Special care is needed when managing patients with diabetes, as a VLCD significantly reduces plasma glucose concentration independently of weight loss. For this reason hypoglycaemia is likely to occur if insulin or sulphonylureas are not reduced or stopped.

  • Reduce by 50% at the beginning of the VLCD if fasting blood glucose >10mmol-1 or HB1Ac is >9-10%.
  • Stop altogether if fasting blood glucose is
Long and Short Acting Insulin
  • OPTIFAST VLCD has approximately 13-22.5 grams of carbohydrate per serve; hence it is recommended that the short acting insulin be reduced to half.
  • A VLCD can rapidly result in a reduction in endogenous glucose production, so the dose of long acting insulin also needs to be reduced, initially by around 50%.

It is imperative that patients are asked to monitor blood glucose levels more frequently for the first few days (at least 4 times per day – once before each meal and before going to bed). For optimum management, it is recommended that patients start the regimen on the weekend, when they can be at home and thus more attentive to the symptoms of hypoglycaemia. It is extremely important that patients diagnosed with Type 1 diabetes continue insulin treatment, no matter how low the insulin dose is, to maintain euglycaemia. For further information on the management of patients with Type 1 & Type 2 diabetes please refer to the OPTIFAST VLCD Co-morbidity Guidelines.


Combined hyperlipidaemia will respond markedly to VLCDs. Therefore it is recommended to stop or decrease medication except in familial hypercholesterolemia.


Diuretics should be stopped at the beginning of the VLCD because VLCDs themselves have a diuretic effect. Other medication for hypertension should be continued and tapered according to blood pressure values.


Most patients on warfarin are suitable for the OPTIFAST VLCD Program but precautions need to be taken. It is recommended to continue with the usual warfarin dose but International Normalised Ratio (INR) levels should be monitored more often and the dose adjusted accordingly.


Patients on lithium may experience changes in serum lithium levels due to sodium depletion and renal retention of lithium. Lithium levels should be monitored weekly, then bimonthly. Lithium may interfere with thyroid function; therefore thyroid function should be checked periodically. Patients using lithium should maintain adequate fluid intake of 2.5 to 3 litres per day and limited xanthine intake is advised. Xanthine is found in caffeine, theophylline and theobromine and in the following beverages and foods: tea, coffee, cola, cocoa, chocolate, and some carbonated drinks.


There is evidence that a ketogenic diet does not alter the blood levels of most anti-epileptic medications, therefore anticonvulsant therapy is not a contraindication to the use of OPTIFAST VLCD. Given that long-term anti-convulsant therapy may result in vitamin D deficiency which may be associated with hypocalcaemia and elevated parathyroid hormone, it is important to check Vitamin D, calcium and parathyroid hormone status prior to starting OPTIFAST VLCD in patients who have had long-term exposure to anticonvulsants.


Chronic use of steroids (more than 20mg daily of prednisolone or its equivalent) must be evaluated carefully because of the tendency to nitrogen waste caused by the drugs. Acute short-term steroid therapy of one to two weeks duration may not be a problem. If the risk/benefit ratio favours treatment, these patients may require more protein to counteract potential catabolic effects of steroid therapy.

Chronic use of drugs with gastro-intestinal (GI) side effects

Drugs with potent GI side effects (for example, non-steroidal anti-inflammatory drugs and steroids) need to be evaluated. If food had a significant buffering effect, a person may require antacids, cimetidine or enteric-coated aspirin to prevent GI side-effects.