Pharmaceutical companies have worked out that the reason different people may react differently to the same drug (i.e. in some people the drug does exactly as intended, in others it doesn't work very well or maybe not at all, & in some it may even cause harmful side effects) is because of their different genetic make up, which causes subtle variations in the target molecules a drug is aimed at. The Human Genome Project means that the pharmaceutical companies now have available to them information which may help them to design drugs for people according to genotype, & the work being carried out in this area is called pharmacogenetics. I'm not a pharmacist, but here is a simplified example of the way I understand it 'could' work: suppose gene X codes for the protein receptor at which codeine is aimed; according to a person's genetic make up, they may have either gene X1, X2, or X3 (variations on the same gene are called alleles). If that person has the X2 allele then the standard codeine that has been available up til now doesn't really do much to relieve their pain; if they have the X3 allele then codeine gives works on the pain but gives them horrible side effects, but if they were lucky enough to have been born with the X1 allele then they get all of the pain relief & none of the side effects. So pharmacogenics will (hopefully) mean that eventually there will be 3 differents kinds of codeine available: codeine1, codeine2, & codeine3, meaning that doctors will be able to prescribe the version best suited to the individual patient.