Do stimulants increase the IQ tests score for everyone? There is some meta-analytic evidence that stimulants increase the IQ test scores of ADHD children by 2 to 7 points (Jepsen et al., 2009). 
Although giving stimulants to non-ADHD children may be unethical, I'm still wondering if there is improvement in IQ test score in children (or adults) who do not meet the criteria for an ADHD diagnosis. An additional motivation for asking this is that a study by Tsai et al. (2013) found that the IQ test score improvement from methylphenidate "had no correlation with the decrement of ADHD symptoms." 


Jepsen, J. R. M., Fagerlund, B., &amp; Mortensen, E. L. (2009). Do attention deficits influence IQ assessment in children and adolescents with ADHD?. Journal of Attention Disorders, 12(6), 551-562.
Tsai, C. S., Huang, Y. S., Wu, C. L., Hwang, F. M., Young, K. B., Tsai, M. H., &amp; Chu, S. M. (2013). Long-term effects of stimulants on neurocognitive performance of Taiwanese children with attention-deficit/hyperactivity disorder. BMC psychiatry, 13(1), 330.
IQ in ADHD sufferers indeed increases with the treatment of stimulants, such as methylphenidate (Meyes et al., 1994). This is thought to rely on the attention-narrowing effect of stimulants, i.e., they cause the 
 easily distracted ADHD child to focus better on the IQ task (Aman, 1996). 

It is an interesting question indeed whether such an effect also plays a role in healthy controls. Indeed it does, at least according to one study, where a control population was subjected to amphetamine. 


Amphetamine improves performance on spatial working memory, Stroop tasks and enhances language production (Barch &amp; Carter, 2005). The authors did, however, not measure performance on full blown IQ tests.


References
- Aman, J Development Phys Disabil (1996); 8(4): 347-65
- Barch &amp; Karter, Schizophrenia Res (2005); 77(1): 43-58
- Meyes et al., DMCN (1994); 36(12): 1099–107