What Is the Difference Between ADHD and ADD?

The Three Types of ADHD

Although ADHD is commonly referred to as a single diagnosis, healthcare professionals distinguish between three different types. These types describe different combinations of symptoms that affect people with ADHD in varying ways. All three fall under the same diagnosis – ADHD – but they differ depending on which symptoms dominate.

• Predominantly hyperactive/impulsive ADHD is characterised by difficulties regulating activity levels, such as restlessness, or impulsive behaviours like interrupting others. This is the least common type, affecting fewer than 15% of people with ADHD.

• Predominantly inattentive ADHD is defined by concentration problems: struggling to maintain or shift focus, losing things, and often seeming “daydreamy”. This type was formerly known as ADD. Today ADD is not used as an official diagnosis, but the term is still widespread in everyday conversation. Roughly 20–30% of people with ADHD fall into this category.

• Combined ADHD is the most common form, present in around 50–75% of cases. It means the individual meets criteria for both inattention and hyperactivity/impulsivity.

For clarity, we will continue to use the term ADD when referring to the predominantly inattentive presentation.

A Shared Neurological Basis

All three types of ADHD are classified under the same diagnosis because they share a neurological foundation. ADHD is a neurodevelopmental condition, which means there are similarities in brain structure and function regardless of subtype.

One key area is executive functions (EF) – the ability to plan, organise, regulate emotions, and inhibit impulses. In the 1990s, theories suggested that EF difficulties were more pronounced in certain types, such as combined ADHD. However, newer research shows that these challenges are common across all types.

Treatment also reflects this common basis. ADHD is most often treated with stimulant medication, regardless of presentation. The effectiveness may vary slightly depending on the type, but the fact that the same medication is prescribed supports the idea that ADHD subtypes share underlying mechanisms. At the same time, individual differences remind us that treatment always needs to be tailored.

Differences in Processing Speed

Processing speed refers to how quickly the brain can take in, process, and use information. Research has shown differences in processing speed between ADHD types:

• Hyperactive/impulsive type often has a faster processing speed. This can be an advantage in situations requiring quick responses, like sports or games. But it can also lead to impulsive mistakes, particularly in tasks that demand accuracy or reflection, such as solving maths problems or filling out forms.

• Inattentive type (ADD) is more likely to experience slower processing speed. This means it may take longer to understand or respond to instructions, especially if tasks are complex or require sustained focus.

• Combined type usually falls in between. Individuals may show some difficulties in cognitively demanding situations, but these are generally less pronounced than in the inattentive type.

These variations in processing speed support the idea that the three types represent distinct presentations of the same condition.

Comorbidity in ADHD – Similarities and Differences

Comorbidity – having more than one condition at the same time – is common across all ADHD types. Shared risks include sleep difficulties, low self-esteem, and depression. This reinforces the shared neurological basis of ADHD.

But research also shows subtype-specific patterns:

• Hyperactive/impulsive and combined types are more vulnerable to conditions linked to impulsive behaviour. These include binge eating, substance misuse, and oppositional defiant disorder (ODD). Here impulsivity is key: acting without reflection and struggling with self-control increases risk. These are known as externalising problems because they are expressed outwardly, through behaviour and conflict.

• Inattentive type (ADD), on the other hand, is more often linked to anxiety disorders. This subtype tends towards internalising problems, where challenges are expressed inwardly as worry, stress, or emotional distress rather than through outward behaviour.

This difference in comorbidity profiles highlights the distinct vulnerability patterns of the ADHD subtypes.

Summary

ADD is not a separate diagnosis but one of three recognised presentations of ADHD. All three share neurological foundations, common comorbidities, and similar treatment approaches. At the same time, differences such as variations in processing speed and distinct comorbidity risks show that these are subtypes of the same condition, not entirely different diagnoses.

Understanding both the similarities and the differences is essential for providing effective support and treatment, and for helping each individual reach their full potential.