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Phentermine Risk-Benefit Assessment - Confidential

1. Objective

In December 2020, the Centre for Adverse Reactions Monitoring (CARM) received a report involving a stroke case in which phentermine was suspected to be the causative medication (CARM ID 139240). The Medsafe pharmacovigilance team reviewed this case with concerns about the safety of phentermine in relation to its clinical benefits, given previous reports of cerebrovascular and cardiovascular events to CARM. It was determined that a risk-benefit evaluation was necessary and that the review would be presented to the Medicine Adverse Reactions Committee (MARC) for guidance.

Additionally, a review of the New Zealand data sheet for Duromine (the approved product containing phentermine) indicates that it suggests continued use of phentermine beyond 12 weeks, although such extended use has not been approved in other countries. Medsafe seeks the advice of the MARC on whether this statement should be revised, contingent upon the Committee's overall assessment of the benefit-risk profile of this medication.

2. Background

2.1 Obesity in New Zealand

According to the 2019/2020 New Zealand Health Survey [1]:

Approximately one in three adults (aged 15 and older) were obese (30.9%).

The prevalence of obesity varied by ethnicity: 63.4% of Pacific, 47.8% of Maori, 29.3% of European/others, and 15.9% of Asian adults were obese.

Adults living in the most socioeconomically deprived areas were 1.8 times more likely to be obese than those in the least deprived areas.

2.2 Clinical Guidelines for Weight Management in New Zealand Adults [2]

In 2017, the Ministry of Health released clinical guidelines on weight management in adults, known as the Guidelines. These guidelines aimed to provide healthcare practitioners with an up-to-date tool for monitoring, assessing, and supporting overweight and obese adults in achieving and maintaining a healthy weight. Excess weight gain can lead to serious health issues, including type 2 diabetes, ischaemic heart disease, stroke, common cancers, osteoarthritis, sleep apnea, and reproductive abnormalities.

The Guidelines outline a four-step continuous process for weight management:

Steps 3 and 4 encompass pharmacological considerations:

Manage:

The use of weight-loss medications is mentioned in this step. The Guidelines recommend considering weight-loss drugs for individuals who:

Have not achieved significant benefit from lifestyle changes after at least six months.

Have a BMI ?30 kg/m2.

People should always use weight loss drugs in conjunction with lifestyle changes. The Guidelines suggest evaluating the efficacy of the weight-loss drug within the first three months of treatment and periodically throughout its use, with careful monitoring for side effects (monthly for the first three months, then quarterly). If an individual on a particular weight-loss medication fails to achieve a weight loss of less than 5% of their initial body weight within 12 weeks, discontinuation of treatment is recommended, except for orlistat, which is approved for long-term use in New Zealand. While other medicines are licensed elsewhere, the two drugs approved for weight loss in New Zealand are orlistat and phentermine. The use of low-dose topiramate in combination with low-dose phentermine for weight loss constitutes an off-label use of topiramate in New Zealand, as topiramate is approved for epilepsy treatment and migraine prophylaxis.

Notably, the Guidelines do not list liraglutide as a pharmacological option, despite its approval for this indication. Naltrexone with bupropion was approved by Medsafe in 2020, which could explain its absence in the Guidelines. Duromine is the sole approved phentermine product in New Zealand, and failure to achieve a 5% weight reduction within 12 weeks is grounds for treatment discontinuation.

Although no studies have confirmed an impact on mortality or morbidity, a weight reduction of 5% or more is considered clinically significant, as it has been associated with reductions in certain obesity-related risk factors.

Maintain:

Once the patient has achieved their desired weight loss, ongoing monitoring and support are necessary. Prescribed weight-loss medications require follow-up care, and their continued use is recommended for weight-loss maintenance only if the patient has lost at least 5% of their initial body weight within the first three months of treatment and side effects remain manageable.

2.3 Bpacnz - Weight Loss: Options and Evidence [4]

In 2019, Bpacnz published an article on weight management, outlining the evidence for various pharmacological and non-pharmacological weight loss options. The advice aligns with the Clinical Guidelines for Weight Management in New Zealand Adults. Regarding pharmacological options for weight loss, Bpacnz mentions:

Pharmacological interventions for weight loss typically have limited long-term effectiveness but may be considered for obese individuals in the short term as an adjunct to lifestyle interventions, following a risk-benefit assessment.

Clinical benefits of weight loss start when an overweight person loses as little as 5% of their body weight, with increasing benefits as they approach their ideal weight range.

Phentermine, a dopaminergic agonist acting as an appetite suppressant, is indicated for short-term (i.e., 12 weeks or less) adjunctive treatment for weight loss in patients with a BMI greater than 30 kg/m2.

Phentermine is contraindicated in several patient groups, especially those with cardiac abnormalities and hypertension.

Like amphetamine, phentermine is a sympathomimetic drug, and concerns exist regarding its potential for addiction.

Bpacnz recommends a four-week trial of phentermine, with consideration of treatment beyond 12 weeks for patients still losing weight, while prescribers should be vigilant for signs of dependence.

2.4 UK's National Institute for Health and Care Excellence (NICE) Guidelines [5]

Pharmacological treatment is recommended only after dietary, exercise, and behavioral approaches have been initiated and evaluated. It may be considered for patients who have not reached their target weight loss or have plateaued despite dietary, activity, and behavioral changes. Pharmacological treatments may be used to maintain weight loss rather than to continue losing weight.

Orlistat: Consider for individuals with a BMI ?28 kg/m2 and associated risk factors or a BMI ?30 kg/m2 or more. Treatment longer than three months should be considered only if the person has lost at least 5% of their initial body weight since starting drug therapy.

Liraglutide - a glucagon-like peptide-1 (GLP-1) receptor agonist.

Naltrexone with bupropion.

2.5 Profiles of Different Pharmacological Treatments for Weight Loss Approved in New Zealand

Table 1 provides a comparison of the efficacy and safety of various weight-loss medications approved in New Zealand. Unlike phentermine, which is indicated for short-term use, other weight-loss medications may be used for longer durations. Orlistat has slightly lower efficacy than other long-term weight-loss medicines. It hinders fat digestion and absorption, leading to notable adverse effects (e.g., faecal urgency, flatulence, cramps, and impaired fat-soluble vitamin absorption). Fewer than 10% of patients continue orlistat treatment for over a year. Orlistat is available without a prescription (pharmacist-only), making it more accessible compared to other options.

Liraglutide seems to offer the most substantial weight loss potential at one year compared to other products. It acts centrally to regulate appetite and has delayed gastric emptying effects. It also enhances glucose homeostasis by reducing fasting and post-meal glucose levels through increased glucose-dependent insulin secretion and decreased glucose-dependent glucagon secretion. However, Liraglutide requires subcutaneous administration and storage between 2-8 degrees Celsius. Common side effects are gastrointestinal, which can be mitigated through gradual dose escalation. Liraglutide has been linked to an increased incidence of symptomatic gallstones and, more rarely, pancreatitis. It is one of the more expensive options.