We included 19 studies that investigated three types of amphetamines: dexamphetamine (10.2 mg/d to 21.8 mg/d), lisdexamfetamine (30 mg/d to 70 mg/d), and mixed amphetamine salts (MAS; 12.5 mg/d to 80 mg/d). These studies enrolled 2521 participants; most were middle-aged (35.3 years), Caucasian males (57.2%), with a combined type of ADHD (78.8%). Eighteen studies were conducted in the USA, and one study was conducted in both Canada and the USA. Ten were multi-site studies. All studies were placebo-controlled, and three also included an active comparator: guanfacine, modafinil, or paroxetine. Most studies had short-term follow-up and a mean study length of 5.3 weeks.
We found no studies that had low risk of bias in all domains of the Cochrane ‘Risk of bias’ tool, mainly because amphetamines have powerful subjective effects that may reveal the assigned treatment, but also because we noted attrition bias, and because we could not rule out the possibility of a carry-over effect in studies that used a cross-over design.
Sixteen studies were funded by the pharmaceutical industry, one study was publicly funded, and two studies did not report their funding sources.
Amphetamines versus placebo
Severity of ADHD symptoms: we found low- to very low-quality evidence suggesting that amphetamines reduced the severity of ADHD symptoms as rated by clinicians (SMD −0.90, 95% confidence interval (CI) −1.04 to −0.75; 13 studies, 2028 participants) and patients (SMD −0.51, 95% CI −0.75 to −0.28; six studies, 120 participants).
Retention: overall, we found low-quality evidence suggesting that amphetamines did not improve retention in treatment (risk ratio (RR) 1.06, 95% CI 0.99 to 1.13; 17 studies, 2323 participants).
Adverse events: we found that amphetamines were associated with an increased proportion of patients who withdrew because of adverse events (RR 2.69, 95% CI 1.63 to 4.45; 17 studies, 2409 participants).
Type of amphetamine: we found differences between amphetamines for the severity of ADHD symptoms as rated by clinicians. Both lisdexamfetamine (SMD −1.06, 95% CI −1.26 to −0.85; seven studies, 896 participants; low-quality evidence) and MAS (SMD −0.80, 95% CI −0.93 to −0.66; five studies, 1083 participants; low-quality evidence) reduced the severity of ADHD symptoms. In contrast, we found no evidence to suggest that dexamphetamine reduced the severity of ADHD symptoms (SMD −0.24, 95% CI −0.80 to 0.32; one study, 49 participants; very low-quality evidence). In addition, all amphetamines were efficacious in reducing the severity of ADHD symptoms as rated by patients (dexamphetamine: SMD −0.77, 95% CI −1.14 to −0.40; two studies, 35 participants; low-quality evidence; lisdexamfetamine: SMD −0.33, 95% CI −0.65 to −0.01; three studies, 67 participants; low-quality evidence; MAS: SMD −0.45, 95% CI −1.02 to 0.12; one study, 18 participants; very low-quality evidence).
Dose at study completion: different doses of amphetamines did not appear to be associated with differences in efficacy.
Type of drug-release formulation: we investigated immediate- and sustained-release formulations but found no differences between them for any outcome.
Amphetamines versus other drugs
We found no evidence that amphetamines improved ADHD symptom severity compared to other drug interventions.