Can you inject Adderall or Ritalin and if not why?

Yes, if you had pure Ritalin it can be injected however injecting Ritalin (methylphenidate) from any tablet without proper purification would be highly dangerous as there are many other ingredients in those tablets which would need to be filtered out.

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While the above answer is correct, despite the fact you will do what you want regardless, some additional information may make one or two people think twice and justify the effort.

If you read the prescribing information for methylphenidate HLC several important points are made. Most important being that methylphenidate HCL is freely soluble in water, and that the drug is acidic. Unless you intend to use a solvent like methanol or acetone which would produce toxic effects, the water solubility is essential. Acidic chemicals tend to have an advantage with regards to removing additional impurities as they will cross between two immiscable solvents if one is made acidic. The impurities remain in the original solute while the product concentrates in the lower pH layer, allowing one to remove the non-pH-lowered solvent to be drained off. Since most persons reading this answer will be using a spoon and water, no point in further discussion there.

While the fillers and other tablet components are unhealthy to inject for various reasons I'll focus on the particulate matter that is introduced directly into the vein. Despite your best efforts to remove these particles, it can't be done without supplies beyond a cotton ball or coffee filter. In a clinical setting you will see patients with irreversible damage to the valves that stop blood from flowing backward when the heart beats. This damage itself causes inefficiency and as a result an increase in cardiac load. In the short-term symptoms such as shortness of breath, fatigue, and edema can be seen. Over time the cumulative effect can be an enlargement of the heart muscle to a point that the organ can no longer effectively circulate blood. *Subaortic stenotic hypertrophy.

Stimulant medications, taken appropriately, carry an increased risk of sudden cardiac death. The leading cause of sudden cardiac death in younger patients is Cardiomyopathy (or Subaortic senotic hypertrophy.) While it often doesn't get that far without diagnosis the outlook isn't good. Medication can manage some aspects to an extent, but only for awhile. Treatment over the longterm requires a transplant, something persons with a history of IV substance abuse are not likely to receive.

My clinical background is shadowed by IV substance abuse, and therefor I can say from an understanding but stern position that anyone taking the risk of injecting drugs in this manner is well past the point of a casual user. Willingness to even consider this route shows desperation. Get help now so we don't waste resources on treating you later.