SNPs may also explain why some patients respond well to a specific drug treatment, while others have minimal or no response. SNPS may also be involved when patients have different side effects in response to the same drug.

Many proteins interact with the drug - involved in its transportation throughout the body, absorption into tissues, metabolism into more active forms or toxic by-products, and excretion. If a patient has SNPs in any one or more of these proteins, they may alter the time the body is exposed to active forms of the drug or any of its toxic byproducts.