According to the DSM-5, a comprehensive medical manual that helps provide diagnostic criteria for a wide array of mental health conditions, bulimia nervosa (usually shortened just to bulimia) has distinct diagnostic criteria. 
Recurrent Binge Eating
A person with bulimia has recurrent episodes of binge eating. Binge eating is a pattern of behavior where a person eats significantly more food in a two-hour period than the average person would in similar circumstances and during that same period.
A person who is binge eating also feels as though they cannot stop eating or can’t control how much they’re eating.
Recurrent Inappropriate Compensatory Behaviors
Compensatory behaviors are behaviors a person engages in to compensate for action, in this case, to compensate for binge eating. People with bulimia engage in what is called inappropriate compensatory behaviors, which are behaviors that are destructive to overall health.
These behaviors can include:
- Self-inducing vomiting
- Misusing laxatives, diuretics, or similar medications
- Excessive exercise
In contrast, appropriate compensatory behaviors might include:
- Talking to a mental health professional to gain more control over eating habits
- Developing a healthy meal plan that can provide all necessary nutrients while allowing the person to stay in their recommended weight range
- Engaging in moderate exercise, taking care not to push the body too hard
Significant Frequency of Occurrence
While a person meeting the above two criteria very likely is struggling with some type of mental health issue, the DSM-5 does note a person must engage, on average, in the above behaviors at least once a week for three months to diagnostically qualify as bulimia.
This is also one of the primary ways the severity of an individual’s bulimia is determined:
- Mild: 1–3 episodes per week
- Moderate: 4–7 episodes per week
- Severe: 8–13 episodes per week
- Extreme: 14 or more episodes per week
Disturbances Not Exclusive to Episodes of Anorexia
This piece of diagnostic criteria is fairly straightforward. Broadly, if a patient’s pattern of behavior better matches that of anorexia nervosa, an anorexia diagnosis is likely more appropriate than a bulimia diagnosis.
The DSM-5’s diagnostic criteria for anorexia nervosa are as follows: 
Restricted Energy Intake
People with anorexia restrict their energy intake, with the unit generally used to measure a person’s energy intake being the calorie.
When a person is taking in fewer calories than their body burns for energy, they experience weight loss. This leads many people with anorexia to reach a significantly low weight, which can cause a number of negative health effects.
The severity of a person’s condition is generally determined by their position on the body mass index (BMI) scale:
- Mild: BMI of 17 or greater
- Moderate: BMI of 16–16.99
- Severe: BMI of 15–15.99
- Extreme: BMI of less than 15
Intense Fear of Weight Gain
A person struggling with anorexia has an intense fear of weight gain and growing fat. This fear can be crippling, with a person struggling with anorexia feeling this fear regardless of their current weight or how much they have eaten in the day.
Distorted Perception of Self
People with anorexia struggle with self-evaluation, often finding it extremely difficult to see themselves as others see them. Many place significant value on weight and body shape and view themselves extremely negatively regardless of their actual current weight or shape.
They also often struggle to understand the severity of their condition and may not recognize the damage a low body weight may have on their health.
Anorexia can be characterized into one of three subtypes:
Restricting-type anorexia is characterized by significant restriction of energy intake but without recurrent binge-eating or purging behavior over the last three months. When individuals discuss anorexia, this is usually the type that they think of as the “default.”
This type of anorexia is characterized by a person engaging in a recurrent pattern of binge eating and purging within the past three months.  It is easily mistaken as bulimia by non-experts.
Notably, a person with this condition still meets the DSM-5 diagnostic criteria for anorexia. They just also have the additional symptoms of binge eating and purging behavior.
Atypical anorexia shares identical symptoms with more typical cases of anorexia, but the patient is at a normal weight range or above. It is often mistaken for a less serious eating disorder or misunderstood as a less dangerous attempt at weight loss than it actually is.
Main Similarities Between Bulimia & Anorexia
Bulimia and anorexia are both eating disorders, and bulimia and purging-type anorexia are both easily mistaken for each other. People struggling with bulimia and anorexia often share many of the same anxieties, commonly suffering from a poor self-image, having serious anxiety around eating, and resorting to unhealthy measures in an attempt to control their weight.
Both conditions need to be taken seriously and can have a detrimental effect on a person’s physical health over time. With these conditions and many other eating disorders, the individual dealing with the disorder may have difficulties admitting they have a problem and seeking help.
Key Differences Between Bulimia & Anorexia
While bulimia can have serious health consequences, anorexia is generally considered the more serious condition of the two eating disorders.
Anorexia has a significant risk of serious medical consequences, with a substantial risk of death that is not typical of many other mental illnesses. While people with bulimia and anorexia share many of the same anxieties and tendencies to develop distorted self-images, these are typically more severe in people with anorexia.
Can a Person Have Both Bulimia and Anorexia?
While the DSM-5 diagnostic criteria for anorexia and bulimia arguably might allow for a double diagnosis, a person exhibiting all the criteria of both is typically going to be diagnosed with binge eating/purging-type anorexia.
While eating disorder research is ongoing and there is much we still don’t know, the recommended treatment for both conditions is similar enough that it seems unlikely a dual diagnosis would serve a significantly different purpose to a binge eating/purging-type anorexia diagnosis.
How to Find Treatment for Anorexia & Bulimia
The first important step for a person with either bulimia or anorexia to make is admitting they have a problem and want to get help. Ideally, a person will do this as early as possible, but it isn’t easy.
Many people have significant trouble even admitting there is an issue until they experience significant medical issues. Family and friends can be an excellent help in this early part of the recovery process, talking to the individual about their concerns and encouraging them to see an expert.
While a number of evidence-based treatments exist for both conditions, the usual first choice is cognitive remediation therapy (CRT). This therapy focuses on helping a person change the way they think so they can become more adaptable and better aware of their own thought patterns. Working with a professional, they will try to shift their negative thoughts about their bodies and food so they can more easily adopt healthier habits.
Some patients with severe symptoms may need residential, or inpatient, treatment. This type of treatment involves staying at a facility and receiving highly focused care for multiple weeks. Treatment like this can help a person physically and mentally recover to the point where they can better function in the outside world. Inpatient care often involves nutritional therapy as well as refeeding strategies. Once stable, they will switch to an outpatient care option, like CRT.
If a treatment option doesn’t feel like it is working for you, talk to your care team. There are a number of alternative therapy options that have been shown to treat eating disorders well. The specific option that works best will depend on your individual needs.