Ethical conduct of research is important not only to safeguard the dignity and respect of the research participants but also to protect the researcher from unnecessary interrogation. There should be justifiable reasons for not taking written consent. In fact there are no any strict measures where the written consent should not be taken. The important thing is Written consent should not make the vulnerable population more vulnerable. 

Doing research in a ethical manner is always important in any topics at the stage of data collection, analysis and dissemination. People raise issue the way researcher project the issue in relation to the informants and information collected and used while preparing and disseminating the research finding. It does not matter either researcher has written or verbal  consents which becomes all the time inadequate. Therefore, more importantly we should have written or verbal consent documented and we must be ethical on our mind and heart  all the time towards the informants during the research process.    

Madhusudan,

There is no 'absolute' answer to this. Consent issues vary between studies and geographies. Many formal ethics committees will initially look for a more 'permanent' written consent where this is reasonably possible. However, often 'implied' consent also applies. For instance, being invited to take part in an online survey and then completing the survey (with relevant preliminary information being provided) implies unwritten consent. It's not so much about risks and benefits - but more what carefully considered requirements need to be in place to best bring about best possible outcomes for both researchers and participants. 

I agree with colleagues that Consent as a requirement must be balanced with other issues sorrounding the research process such as socio-cultural and geographic context, and the research approach. Beyond consent, whether written or verbal, explicit or implied, what is important (I find) is that study participants are adequately offered enough background information regarding the research to enable them make an "informed" decision whether to participate or not. Spefic answers may also depend with the ethics review board and exemptions are possible depending on the circumstances, but the ultimately need is to protect participants from harm or coercion.

Thank you all for your views and experiences.