NAIROBI, Kenya (AP) _ An international conference on the marketing and use of drugs opens Monday, with lobbyists for consumer groups and drug manufacturers expected to join the experts.

The World Health Organization - WHO - has invited about 100 specialists to participate in the five-day conference to discuss the promotion of proper uses of drugs and marketing and distribution in developing countries.

Consumer groups are urging greater involvement by WHO in regulating the international flow of harmful or useless drugs. The pharmaceutical industry opposes any type of WHO-monitored marketing code and says it own code, adopted in 1981, is effective.

Names of the participants were not released, but WHO said they include representatives of governments, drug regulatory agencies, drug manufacturers, medical professional groups and consumer organizations.

The conference was authorized in May 1984 by the World Health Assembly, WHO's policy-making body.

WHO, affilated with the United Nations and based in Geneva, Switzerland, says no resolutions or policy recommendations will be adopted at the conference. Instead, the discussions are to serve as guidance for WHO's director-general, Dr. Halfdan Mahler of Denmark, in drafting a report for the World Health Assembly's annual meeting next year.

Squaring off on the sidelines of the conference are the International Federation of Pharmaceutical Manufacturers Associations and a coalition of consumer groups called Health Action International.

Health Action International, in a news release, claimed that up to 70 percent of the medicines on the world market are inessential or undesirable. It said WHO has compiled a list of 220 ''essential'' drugs that would meet most health needs, yet many countries have registered between 10,000 and 20,000 drug brand names.

WHO has expressed concern about inappropriate and excessive prescription and use of drugs, but it suggests that individual governments should take the lead in seeking remedies.

The drug manufacturers federation has said the campaign for tighter regulation is an ideologically motivated attack on Western multinational corporations.

A federation position paper said, ''It would be a tragedy should misguided attacks lead to onerous regulations in the form of a WHO code that, by confusing issues of ethics and economics, would reduce the flow of new medicines for tropical diseases... and ultimately hurt the very people the code was intended to protect.''

Consumer groups have accused drug manufacturers of dumping unsafe products, using misleading labels and charging excessive prices in developing countries. The manufacturers' federation has denied any systematic abuses and says drugs are the most efficient form of medical care in countries with few hospitals and doctors.