I am not sure what 'effective' could mean here since the answer can differ from context to context. Sometime fast is effective and sometimes slow is effective. Similarly sometimes a spinal injection is effective and sometime an Intra-arterial injection is effective. So rather than answering your question in context of being universally effective, I will answer it in the context of commonly used effective methods of drug administration. Here are a few ways:
- Most common way for administration of medication is orally
- Usually the fastest way to get a drug to work is to administer it Intravenously (IV)
- Inhaling (like nasal sprays) are also a fast way to get the drugs to work (faster than oral but slower than IV)
- Rectal administration is a slow way to get the drug to work (usually prescribed when oral and IV is not possible)
- Transdermal is basically by a patch of medicated material that releases the drug through skin contact (usually designed to allow for a slow but a continuous release of medication)
Note: These are just a generalization of commonly occurring clinical practices.