I am a law school graduate. What I offer is mere information, not to be construed as forming an attorney client relationship.
This is based on a law professor specializing in immigration law:
Controlled substance violations: An alien may be removed if at any time after admission he or she is convicted of a violation (or a conspiracy or attempt to violate) any law or regulation of a state, the United States, or a foreign country relating to a controlled substance, other than a single offense involving possession for one's own use of 30 grams or less of marijuana. (INA §237(a)(2)(B)(i); 8 USC 1227(a)(2)(B)(i).) A non-citizen who is, or at any time after admission has been, a drug abuser or addict is also removable. (INA §237(a)(2)(B)(ii); 8 USC 1227 (a)(2)(B)(ii).) In addition, illicit trafficking in a controlled substance qualifies as an aggravated felony for immigration purposes. (INA §101(a)(43); 8 USC 1101(a)(43).)
Check the INS website for an exact answer to your question.
I hope this helps.