The "rule of 5" states that: poor absorption or permeation is more likely when:
1. There are more than 5 H-bond donors (expressed as the sum of OHs and NHs);
2. The MWT is over 500;
3. The LogP is over 5 (or MLogP is over 4.15);
4. There are more than 10 H-bond acceptors (expressed as the sum of Ns and Os).
Compound classes that are substrates for biological transporters are exceptions to the rule.
The rule of 5 has been extensively used in the pharmaceutical industry as a way to screen out candidates at the early stage of drug discovery. However, it is well known that compounds which violate these rules can still be very successful as drugs.